Demand driven capacitor charging for cardiac pacing

ABSTRACT

An implantable medical device system delivers a pacing pulse to a patient&#39;s heart and starts a first pacing interval corresponding to a pacing rate in response to the delivered pacing pulse. The system charges a holding capacitor to a pacing voltage amplitude during the first pacing interval. The system detects an increased intrinsic heart rate that is at least a threshold rate faster than the current pacing rate from a cardiac electrical signal received by a sensing circuit of the implantable medical device. The system starts a second pacing interval in response to an intrinsic cardiac event sensed from the cardiac electrical signal and withholds charging of the holding capacitor for at least a portion of the second pacing interval in response to detecting the increased intrinsic heart rate.

RELATED APPLICATION

This application is a Division of U.S. patent application Ser. No.15/676,066, filed Aug. 14, 2017, the content of both of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The disclosure relates generally to an implantable medical device (IMD)system and method that delivers cardiac pacing pulses and particularlyto an IMD system and method for controlling the charging of capacitorsused for generating and delivering cardiac pacing pulses based on pacingdemand.

BACKGROUND

Medical devices, such as cardiac pacemakers and implantable cardioverterdefibrillators (ICDs), provide therapeutic electrical stimulation to aheart of a patient via electrodes carried by one or more medicalelectrical leads and/or electrodes on a housing of the medical device.The electrical stimulation may include cardiac pacing pulses orcardioversion/defibrillation (CV/DF) shocks.

The medical device may sense cardiac electrical events attendant to theintrinsic heart activity for detecting an abnormal intrinsic heartrhythm. Upon detection of an abnormal rhythm, such as bradycardia,tachycardia or fibrillation, an appropriate electrical stimulationtherapy may be delivered to restore or maintain a more normal rhythm ofthe heart. For example, an ICD may deliver pacing pulses to the heart ofthe patient upon detecting bradycardia or tachycardia or deliver CV/DFshocks to the heart upon detecting tachycardia or fibrillation.

The ICD may sense the cardiac electrical signals from a heart chamberand deliver electrical stimulation therapies to the heart chamber usingendocardial electrodes carried by transvenous medical electrical leads.In other cases, a non-transvenous lead may be coupled to the ICD, inwhich case the ICD may sense cardiac electrical signals and deliverelectrical stimulation therapy to the heart using extra-cardiovascularelectrodes. The energy of a therapeutic electrical stimulation pulserequired to effectively stimulate the heart using theextra-cardiovascular electrodes is typically greater than the energyrequired to stimulate the heart using endocardial electrodes. A pacingcircuit may include a holding capacitor that is charged to a pacingvoltage amplitude for generating a pacing pulse according to the pacingpulse energy required to capture the pacing heart using the pacingelectrode vector that is available.

SUMMARY

In general, the disclosure is directed to techniques for controllingcharging of at least one holding capacitor that is used to deliver acardiac electrical stimulation pulse by a therapy delivery circuit of animplantable medical device. An IMD operating according to thesetechniques may withhold capacitor charging when increased intrinsicheart rate criteria are satisfied. Charging of the capacitor may bewithheld for at least a portion of a pacing interval, e.g., by chargingafter a delay interval has expired. The charging delay interval may beequal to, greater than or less than the pacing interval. In response todecreased heart rate criteria being satisfied, the IMD may switch backto charging the holding capacitor without delay, e.g., at the beginningor throughout a pacing interval as needed to maintain the holdingcapacitor charge at the pacing voltage amplitude in a ready state fordelivering a pacing pulse. In some examples, the IMD may be configuredto control when the function of switching between two different chargingmodes, e.g., a delayed capacitor charging mode and a capacitor chargingwithout delay mode, is enabled (turned on) or disabled (turned off).When this charging mode switching function is disabled, the IMD mayoperate to charge the holding capacitor according to one, defaultcharging mode. When the charging mode switching function is enabled, theIMD may operate to switch between two different charging modes based onintrinsic heart rate criteria and/or other pacing demand criteria.

In one example, the disclosure provides an IMD system including atherapy delivery circuit, a sensing circuit and a control circuitcoupled to the therapy delivery circuit and the sensing circuit. Thetherapy delivery circuit has a holding capacitor and a charging circuitconfigured to charge the holding capacitor to a pacing voltageamplitude. The sensing circuit is configured to receive a cardiacelectrical signal from a patient's heart. The control circuit isconfigured to control the therapy delivery circuit to deliver a pacingpulse, start a first pacing interval corresponding to a pacing rate inresponse to the delivered pacing pulse, control the therapy deliverycircuit to charge the holding capacitor during the first pacing intervalaccording to a first charging mode, detect an increased intrinsic heartrate from the cardiac electrical signal that is at least a thresholdrate faster than the pacing rate, switch from the first charging mode toa second charging mode in response to detecting the increased intrinsicheart rate, start a second pacing interval in response to an intrinsiccardiac event sensed from the cardiac electrical signal, and control thetherapy delivery circuit to withhold charging of the holding capacitorfor at least a portion of the second pacing interval according to thesecond charging mode.

In another example, the disclosure provides a method includingdelivering a pacing pulse by a therapy delivery circuit having a holdingcapacitor and a charging circuit configured to the charge the holdingcapacitor to a pacing voltage amplitude and starting a first pacinginterval corresponding to a pacing rate in response to the deliveredpacing pulse. The method further includes charging the holding capacitorduring the first pacing interval according to a first charging mode anddetecting an increased intrinsic heart rate that is at least a thresholdrate faster than the pacing rate from a cardiac electrical signalreceived by a sensing circuit, switching from the first charging mode toa second charging mode in response to detecting the increased intrinsicheart rate, starting a second pacing interval in response to a firstintrinsic cardiac event sensed from the cardiac electrical signal; andwithholding charging of the holding capacitor for at least a portion ofthe second pacing interval according to the second charging mode.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control circuit of an implantable medical device,cause the device to deliver a pacing pulse by a therapy delivery circuithaving a holding capacitor and a charging circuit configured to thecharge the holding capacitor to a pacing voltage amplitude and start afirst pacing interval corresponding to a pacing rate in response to thedelivered pacing pulse. The instructions further cause the device tocharge the holding capacitor during the first pacing interval accordingto a first charging mode, detect an increased intrinsic heart rate thatis at least a threshold rate faster than the pacing rate from a cardiacelectrical signal received by a sensing circuit, switch from the firstcharging mode to a second charging mode in response to detecting theincreased intrinsic heart rate, start a second pacing interval inresponse to a first intrinsic cardiac event sensed from the cardiacelectrical signal; and withhold charging of the holding capacitor for atleast a portion of the second pacing interval according to the secondcharging mode.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem according to one example.

FIGS. 2A-2C are conceptual diagrams of a patient implanted with theextra-cardiovascular ICD system of FIG. 1A in a different implantconfiguration.

FIG. 3 is a schematic diagram of the ICD of FIGS. 1A-2C according to oneexample.

FIG. 4 is diagram of a high voltage therapy circuit of the ICD of FIG. 3according to one example.

FIG. 5 is a diagram of a low voltage therapy circuit of the ICD of FIG.3 according to one example.

FIG. 6 is a flow chart of a method for controlling capacitor chargingfor pacing pulse delivery according to one example.

FIG. 7 is a flow chart of a method for controlling holding capacitorcharging based on intrinsic heart rate criteria according to oneexample.

FIGS. 8A through 8D are timing diagrams depicting operations performedby an ICD in controlling holding capacitor charging based on the timingof sensed intrinsic events.

FIGS. 9A-9C are timing diagrams depicting operations performed by an ICDor pacemaker for withholding capacitor charging according to a delayedcapacitor charging mode and switching back to a charging without delay.

FIG. 10 is a flow chart of a method for controlling capacitor chargingaccording to yet another example.

FIG. 11 is a flow chart of a method for controlling the capacitorcharging mode based on the rate or slope of a change in heart rate.

FIG. 12 is flow chart of a method for controlling capacitor charging forcardiac pacing according to another example.

FIG. 13 is a flow chart of a method for controlling capacitor chargingbased on different pacing therapies according to one example.

FIG. 14 is a diagram of another IMD system that may be configured tocontrol capacitor charging for pacing therapy delivery using thetechniques disclosed herein.

FIG. 15 is a flow chart of a method for controlling holding capacitorcharging according to yet another example.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for controllingcharging of a holding capacitor in a therapy delivery circuit of acardiac medical device or system. A holding capacitor, or a combinationof holding capacitors, is generally charged by a charging circuit to apacing voltage amplitude for generating and delivering a cardiac pacingpulse. The holding capacitor may be charged as needed at the beginningor throughout a pacing interval that is started immediately after apacing pulse or sensed intrinsic cardiac event, such as an R-wave orP-wave. In this way, the holding capacitor is maintained at the pacingvoltage amplitude in a ready state until a pacing timing intervalexpires. Maintaining the charge of a holding capacitor or combination ofcapacitors at the pacing voltage amplitude, however, consumes energysupplied by the power source of the IMD. The techniques disclosed hereinmay be used to conserve energy normally required to charge and maintaina holding capacitor in a ready state for delivering cardiac pacingpulses by withholding capacitor charging during at least a portion orall of a pacing interval when increased heart rate criteria are metand/or low pacing demand criteria are met.

In some examples, the cardiac medical device system implementing thetechniques disclosed herein may be an extra-cardiovascular ICD system.As used herein, the term “extra-cardiovascular” refers to a positionoutside the blood vessels and heart of a patient. Implantable electrodescarried by extra-cardiovascular leads may be positionedextra-thoracically (outside the ribcage and sternum, e.g.,subcutaneously) or intra-thoracically (beneath the ribcage or sternum,e.g., substernally) but generally not in intimate contact withmyocardial tissue. The techniques disclosed herein for controllingcapacitor charging may be applied to a therapy delivery circuit that iscoupled to extra-cardiovascular electrodes and likely to requirerelatively higher pacing voltage amplitude and/or longer pulse widthsthan a cardiac medical device coupled to endocardial or epicardialelectrodes.

As such, techniques disclosed herein are described in conjunction withan ICD and implantable medical lead carrying extra-cardiovascularelectrodes, but aspects disclosed herein may be utilized in conjunctionwith other cardiac medical devices or systems. For example, thetechniques for controlling capacitor charging as described inconjunction with the accompanying drawings may be implemented in anyimplantable or external medical device enabled for delivering cardiacelectrical stimulation pulses, including implantable pacemakers or ICDscoupled to transvenous, pericardial or epicardial leads carrying sensingand therapy delivery electrodes; intra-cardiac or leadless pacemakers orICDs having housing-based electrodes; or external or wearable pacemakersor defibrillators coupled to external, surface or skin electrodes.

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem 10 according to one example. FIG. 1A is a front view of ICDsystem 10 implanted within patient 12. FIG. 1B is a side view of ICDsystem 10 implanted within patient 12. ICD system 10 includes an ICD 14connected to an extra-cardiovascular electrical stimulation and sensinglead 16. FIGS. 1A and 1B are described in the context of an ICD system10 capable of providing defibrillation and/or cardioversion shocks andpacing pulses.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as an electrode (sometimes referred to as a “can”electrode). Housing 15 may be used as an active can electrode for use indelivering cardioversion/defibrillation (CV/DF) shocks or otherelectrical pulses including pacing pulses that may be delivered using ahigh voltage therapy circuit. In other examples, housing 15 may beavailable for use in delivering unipolar, cardiac pacing pulses from alow voltage therapy circuit and/or for sensing cardiac electricalsignals in combination with electrodes carried by lead 16. In otherinstances, the housing 15 of ICD 14 may include a plurality ofelectrodes on an outer portion of the housing. The outer portion(s) ofthe housing 15 functioning as an electrode(s) may be coated with amaterial, such as titanium nitride, e.g., for reducing post-stimulationpolarization artifact.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memories, transceivers,electrical cardiac signal sensing circuitry, therapy delivery circuitry,power sources and other components for sensing cardiac electricalsignals, detecting a heart rhythm, and controlling and deliveringelectrical stimulation pulses to treat an abnormal heart rhythm.Elongated lead body 18 has a proximal end 27 that includes a leadconnector (not shown) configured to be connected to ICD connectorassembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead body 18 includes defibrillation electrodes 24 and 26and pace/sense electrodes 28 and 30. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beactivated independently.

Electrodes 24 and 26 (and in some examples housing 15) are referred toherein as defibrillation electrodes because they are utilized,individually or collectively, for delivering high voltage stimulationtherapy (e.g., cardioversion or defibrillation shocks). Electrodes 24and 26 may be elongated coil electrodes and generally have a relativelyhigh surface area for delivering high voltage electrical stimulationpulses compared to pacing and sensing electrodes 28 and 30. However,electrodes 24 and 26 and housing 15 may also be utilized to providepacing functionality, sensing functionality or both pacing and sensingfunctionality in addition to or instead of high voltage stimulationtherapy. In this sense, the use of the term “defibrillation electrode”herein should not be considered as limiting the electrodes 24 and 26 foruse in only high voltage cardioversion/defibrillation shock therapyapplications. For example, electrodes 24 and 26 may be used in a sensingvector used to sense cardiac electrical signals and detect anddiscriminate abnormal rhythms such as asystole, bradycardia, non-sinustachycardia or fibrillation and/or used in a pacing electrode vector fordelivering cardiac pacing pulses to heart 8.

Electrodes 28 and 30 are relatively smaller surface area electrodeswhich are available for use in sensing electrode vectors for sensingcardiac electrical signals and may be used for delivering pacing pulsesin some configurations. Electrodes 28 and 30 are referred to aspace/sense electrodes because they are generally configured for use inlow voltage applications, e.g., used as either a cathode or anode fordelivery of pacing pulses and/or sensing of cardiac electrical signals,as opposed to delivering high voltage cardioversion/defibrillationshocks. In some instances, electrodes 28 and 30 may provide only pacingfunctionality, only sensing functionality or both. ICD 14 may obtaincardiac electrical signals corresponding to electrical activity of heart8 via one or more sensing vectors that include combinations ofelectrodes 24, 26, 28 and/or 30. In some examples, housing 15 of ICD 14is used in combination with one or more of electrodes 24, 26, 28 and/or30 in a sensing electrode vector.

In the example illustrated in FIGS. 1A and 1B, electrode 28 is locatedproximal to defibrillation electrode 24, and electrode 30 is locatedbetween defibrillation electrodes 24 and 26. Electrodes 28 and 30 may bepositioned at other locations along lead body 18 and are not limited tothe positions shown. Fewer or more pace/sense electrodes may be carriedby lead 16. For instance, a third pace/sense electrode may be locateddistal to defibrillation electrode 26 in some examples. Electrodes 28and 30 are illustrated as ring electrodes; however, electrodes 28 and 30may comprise any of a number of different types of electrodes, includingring electrodes, short coil electrodes, hemispherical electrodes,directional electrodes, segmented electrodes, or the like. In theexample shown, lead 16 extends subcutaneously or submuscularly over theribcage 32 medially from the connector assembly 27 of ICD 14 toward acenter of the torso of patient 12, e.g., toward xiphoid process 20 ofpatient 12. At a location near xiphoid process 20, lead 16 bends orturns and extends superior subcutaneously or submuscularly over theribcage and/or sternum, substantially parallel to sternum 22. Althoughillustrated in FIG. 1A as being offset laterally from and extendingsubstantially parallel to sternum 22, the distal portion 25 of lead 16may be implanted at other locations, such as over sternum 22, offset tothe right or left of sternum 22, angled laterally from sternum 22 towardthe left or the right, or the like. Alternatively, lead 16 may be placedalong other subcutaneous or submuscular paths. The path ofextra-cardiovascular lead 16 may depend on the location of ICD 14, thearrangement and position of electrodes carried by the lead body 18,and/or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, and 30 locatedalong the distal portion 25 of the lead body 18. The elongatedelectrical conductors contained within the lead body 18 are eachelectrically coupled with respective defibrillation electrodes 24 and 26and pace/sense electrodes 28 and 30, which may be separate respectiveinsulated conductors within the lead body 18. The respective conductorselectrically couple the electrodes 24, 26, 28, and 30 to circuitry, suchas a therapy delivery circuit and/or a sensing circuit, of ICD 14 viaconnections in the connector assembly 17, including associatedelectrical feedthroughs crossing housing 15. The electrical conductorstransmit therapy from a therapy delivery circuit within ICD 14 to one ormore of defibrillation electrodes 24 and 26 and/or pace/sense electrodes28 and 30 and transmit sensed electrical signals from one or more ofdefibrillation electrodes 24 and 26 and/or pace/sense electrodes 28 and30 to the sensing circuit within ICD 14.

The lead body 18 of lead 16 may be formed from a non-conductivematerial, including silicone, polyurethane, fluoropolymers, mixturesthereof, and other appropriate materials, and shaped to form one or morelumens within which the one or more conductors extend. Lead body 18 maybe tubular or cylindrical in shape. In other examples, the distalportion 25 (or all of) the elongated lead body 18 may have a flat,ribbon or paddle shape. Lead body 18 may be formed having a preformeddistal portion 25 that is generally straight, curving, bending,serpentine, undulating or zig-zagging.

In the example shown, lead body 18 includes a pre-formed curving distalportion 25 having two “C” shaped curves, which together may resemble theGreek letter epsilon, “ϵ” Defibrillation electrodes 24 and 26 are eachcarried by one of the two respective C-shaped portions of the lead bodydistal portion 25. The two C-shaped curves are seen to extend or curvein the same direction away from a central axis of lead body 18, alongwhich pace/sense electrodes 28 and 30 are positioned. Pace/senseelectrodes 28 and 30 may, in some instances, be approximately alignedwith the central axis of the straight, proximal portion of lead body 18such that mid-points of defibrillation electrodes 24 and 26 arelaterally offset from pace/sense electrodes 28 and 30.

Other examples of extra-cardiovascular leads including one or moredefibrillation electrodes and one or more pacing and sensing electrodescarried by curving, serpentine, undulating or zig-zagging distal portionof the lead body 18 that may be implemented with the techniquesdescribed herein are generally disclosed in pending U.S. Pat.Publication No. 2016/0158567 (Marshall, et al.), incorporated herein byreference in its entirety. The techniques disclosed herein are notlimited to any particular lead body design, however. In other examples,lead body 18 is a flexible elongated lead body without any pre-formedshape, bends or curves. Various example configurations ofextra-cardiovascular leads and electrodes and dimensions that may beimplemented in an IMD system employing the techniques disclosed hereinare described in pending U.S. Publication No. 2015/0306375 (Marshall, etal.) and pending U.S. Publication No. 2015/0306410 (Marshall, et al.),both of which are incorporated herein by reference in their entirety.

ICD 14 analyzes the cardiac electrical signals received from one or moresensing electrode vectors to monitor for abnormal rhythms, such asasystole, bradycardia, or tachyarrhythmias. ICD 14 may be configured toset pacing intervals for timing the delivery of cardiac pacing pulsesaccording to programmed pacing therapy control parameters. ICD 14 may beconfigured to operate according to multiple pacing modes, e.g., VVI(R),VDI(R), VVO(R), etc., and set the pacing timing intervals accordingly.ICD 14 delivers a cardiac pacing pulse in response to a pacing timinginterval expiring. For example, when a ventricular pacing intervalexpires without sensing an intrinsic R-wave during the pacing interval,ICD 14 delivers a pacing pulse to maintain at least a programmed minimumheart rate or provide back-up pacing during asystole, e.g., following aCV/DF shock. Cardiac pacing pulses may be delivered using defibrillationelectrodes 24 and 26 as an anode and cathode pair, using pacing andsensing electrodes 28 and 30 as an anode and cathode pair, or one ofpace/sense electrodes 28 or 30 paired with one of defibrillationelectrodes 24 or 26, or any one of electrodes 24, 26, 28 or 30 pairedwith housing 15.

ICD 14 may also be configured to deliver electrical stimulation therapyin response to detecting a tachyarrhythmia (e.g., VT or VF) using atherapy delivery electrode vector which may be selected from any of theavailable electrodes 24, 26, 28 30 and/or housing 15. ICD 14 may analyzethe heart rate and morphology of the cardiac electrical signals tomonitor for tachyarrhythmia in accordance with any of a number oftachyarrhythmia detection techniques. One example technique fordetecting tachyarrhythmia is described in U.S. Pat. No. 7,761,150(Ghanem, et al.), incorporated herein by reference in its entirety. ICD14 may deliver anti-tachycardia pacing (ATP) in response to VT detectionand in some cases may deliver ATP prior to a CV/DF shock or during highvoltage capacitor charging in an attempt to avert the need fordelivering a CV/DF shock. If ATP does not successfully terminate VT orwhen VF is detected, ICD 14 may deliver one or more CV/DF shocks via oneor both of defibrillation electrodes 24 and 26 and/or housing 15.

FIGS. 1A and 1B are illustrative in nature and should not be consideredlimiting of the practice of the techniques disclosed herein. ICD 14 isshown implanted subcutaneously on the left side of patient 12 along theribcage 32. ICD 14 may, in some instances, be implanted between the leftposterior axillary line and the left anterior axillary line of patient12. ICD 14 may, however, be implanted at other subcutaneous orsubmuscular locations in patient 12. For example, ICD 14 may beimplanted in a subcutaneous pocket in the pectoral region. In this case,lead 16 may extend subcutaneously or submuscularly from ICD 14 towardthe manubrium of sternum 22 and bend or turn and extend inferiorly fromthe manubrium to the desired location subcutaneously or submuscularly.In yet another example, ICD 14 may be placed abdominally. Lead 16 may beimplanted in other extra-cardiovascular locations as well. For instance,as described with respect to FIGS. 2A-2C, the distal portion 25 of lead16 may be implanted underneath the sternum/ribcage in the substernalspace.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with ICD 14 for transmitting and receiving data viacommunication link 42. Communication link 42 may be established betweenICD 14 and external device 40 using a radio frequency (RF) link such asBLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) orother RF or communication frequency bandwidth or protocol.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may be used to program cardiac eventsensing parameters (e.g., R-wave sensing parameters), cardiac rhythmdetection parameters (e.g., VT and VF detection parameters and SVTdiscrimination parameters) and therapy control parameters used by ICD14. Data stored or acquired by ICD 14, including physiological signalsor associated data derived therefrom, results of device diagnostics, andhistories of detected rhythm episodes and delivered therapies, may beretrieved from ICD 14 by external device 40 following an interrogationcommand. External device 40 may alternatively be embodied as a homemonitor or hand held device.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted withextra-cardiovascular ICD system 10 in a different implant configurationthan the arrangement shown in FIGS. 1A-1B. FIG. 2A is a front view ofpatient 12 implanted with ICD system 10. FIG. 2B is a side view ofpatient 12 implanted with ICD system 10. FIG. 2C is a transverse view ofpatient 12 implanted with ICD system 10. In this arrangement,extra-cardiovascular lead 16 of system 10 is implanted at leastpartially underneath sternum 22 of patient 12. Lead 16 extendssubcutaneously or submuscularly from ICD 14 toward xiphoid process 20and at a location near xiphoid process 20 bends or turns and extendssuperiorly within anterior mediastinum 36 in a substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22(see FIG. 2C). The distal portion 25 of lead 16 may extend along theposterior side of sternum 22 substantially within the loose connectivetissue and/or substernal musculature of anterior mediastinum 36. A leadimplanted such that the distal portion 25 is substantially withinanterior mediastinum 36, may be referred to as a “substernal lead.”

In the example illustrated in FIGS. 2A-2C, lead 16 is locatedsubstantially centered under sternum 22. In other instances, however,lead 16 may be implanted such that it is offset laterally from thecenter of sternum 22. In some instances, lead 16 may extend laterallysuch that distal portion 25 of lead 16 is underneath/below the ribcage32 in addition to or instead of sternum 22. In other examples, thedistal portion 25 of lead 16 may be implanted in otherextra-cardiovascular, intra-thoracic locations, including the pleuralcavity or around the perimeter of or within the pericardium 38 of heart8. Other implant locations and lead and electrode arrangements that maybe used in conjunction with the capacitor charging techniques describedherein are generally disclosed in the above-incorporated references.

FIG. 3 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asan electrode in FIG. 3) includes software, firmware and hardware thatcooperatively monitor cardiac electrical signals, determine when anelectrical stimulation therapy is necessary, and deliver electricalstimulation therapies as needed according to programmed therapy deliveryalgorithms and control parameters. ICD 14 is coupled to anextra-cardiovascular lead, such as lead 16 carrying extra-cardiovascularelectrodes 24, 26, 28, and 30, for delivering electrical stimulationpulses to the patient's heart and for sensing cardiac electricalsignals.

ICD 14 includes a control circuit 80, memory 82, therapy deliverycircuit 84, sensing circuit 86, and telemetry circuit 88. In someexamples, ICD 14 includes one or more sensors 90 for producing a signalthat is correlated to a physiological function, state or condition ofthe patient. A power source 98 provides power to the circuitry of ICD14, including each of the components 80, 82, 84, 86, 88 and 90 asneeded. Power source 98 may include one or more energy storage devices,such as one or more rechargeable or non-rechargeable batteries. Theconnections between power source 98 and each of the other components 80,82, 84, 86 and 88 are to be understood from the general block diagram ofFIG. 3, but are not shown for the sake of clarity. For example, powersource 98 is coupled to one or more charging circuits included intherapy delivery circuit 84 for providing the power needed to chargeholding capacitors included in therapy delivery circuit 84 that aredischarged at appropriate times under the control of control circuit 80for producing electrical stimulation pulses according to a therapyprotocol, such as for bradycardia pacing, post-shock pacing, ATP andCV/DF shock pulses. Power source 98 is also coupled to components ofsensing circuit 86, such as sense amplifiers, analog-to-digitalconverters, switching circuitry, etc., sensors 90, telemetry circuit 88and memory 82 to provide power to various circuits or components asneeded.

The functional blocks shown in FIG. 3 represent functionality includedin ICD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to ICD 14 herein. The variouscomponents may include an application specific integrated circuit(ASIC), an electronic circuit, a processor (shared, dedicated, or group)and memory that execute one or more software or firmware programs, acombinational logic circuit, state machine, or other suitable componentsor combinations of components that provide the described functionality.The particular form of software, hardware and/or firmware employed toimplement the functionality disclosed herein will be determinedprimarily by the particular system architecture employed in the ICD andby the particular detection and therapy delivery methodologies employedby the ICD. Providing software, hardware, and/or firmware to accomplishthe described functionality in the context of any modern cardiac medicaldevice system, given the disclosure herein, is within the abilities ofone of skill in the art.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such asrandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control circuit 80and/or other ICD components to perform various functions attributed toICD 14 or those ICD components. The non-transitory computer-readablemedia storing the instructions may include any of the media listedabove.

The functions attributed to ICD 14 herein may be embodied as one or moreintegrated circuits. Depiction of different features as circuits isintended to highlight different functional aspects and does notnecessarily imply that such circuits must be realized by separatehardware or software components. Rather, functionality associated withone or more circuits may be performed by separate hardware, firmware orsoftware components, or integrated within common hardware, firmware orsoftware components. For example, therapy control operations fordelivering electrical stimulation pulses may be performed cooperativelyby therapy delivery circuit 84 and control circuit 80 and may includeoperations implemented in a processor or other signal processingcircuitry included in control circuit 80 executing instructions storedin memory 82. These therapy control operations may include controllingwhen holding capacitor charging to a pacing voltage amplitude isperformed according to capacitor charging management techniquesdisclosed herein.

Control circuit 80 may include fixed function circuitry and/orprogrammable processing circuitry. Control circuit 80 may include anyone or more of a microprocessor, a controller, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield-programmable gate array (FPGA), or equivalent discrete or analoglogic circuitry. In some examples, control circuit 80 may includemultiple components, such as any combination of one or moremicroprocessors, one or more controllers, one or more DSPs, one or moreASICs, or one or more FPGAs, as well as other discrete or integratedlogic circuitry. The functions attributed to control circuit 80 hereinmay be embodied as software, firmware, hardware or any combinationthereof.

Control circuit 80 communicates, e.g., via a data bus, with therapydelivery circuit 84 and sensing circuit 86 for sensing cardiacelectrical activity, detecting cardiac rhythms, and controlling deliveryof cardiac electrical stimulation therapies in response to sensedcardiac signals. Therapy delivery circuit 84 and sensing circuit 86 areelectrically coupled to electrodes 24, 26, 28, 30 carried by lead 16 andthe housing 15, which may function as a common or ground electrode or asan active can electrode for delivering CV/DF shock pulses or cardiacpacing pulses.

Sensing circuit 86 may be selectively coupled to electrodes 28, 30and/or housing 15 in order to monitor electrical activity of thepatient's heart. Sensing circuit 86 may additionally be selectivelycoupled to defibrillation electrodes 24 and/or 26 for use in a sensingelectrode vector together or in combination with one or more ofelectrodes 28, 30 and/or housing 15. Sensing circuit 86 may be enabledto selectively receive cardiac electrical signals from one or moresensing electrode vectors from the available electrodes 24, 26, 28, 30,and housing 15. Sensing circuit 86 may monitor one or more cardiacelectrical signals at a time for sensing cardiac electrical events,e.g., P-waves attendant to the depolarization of the atrial myocardiumand/or R-waves attendant to the depolarization of the ventricularmyocardium, and providing digitized cardiac signal waveforms foranalysis by control circuit 80. For example, sensing circuit 86 mayinclude switching circuitry for selecting which of electrodes 24, 26,28, 30, and housing 15 are coupled to cardiac event detection circuitry.Switching circuitry may include a switch array, switch matrix,multiplexer, or any other type of switching device suitable toselectively couple components of sensing circuit 86 to selectedelectrodes.

The cardiac event detection circuitry may be configured to amplify,filter and digitize the cardiac electrical signal received from selectedelectrodes to improve the signal quality for detecting cardiacelectrical events, such as R-waves or performing other signal analysis.The cardiac event detection circuitry within sensing circuit 86 mayinclude one or more sense amplifiers, filters, rectifiers, thresholddetectors, comparators, analog-to-digital converters (ADCs), timers orother analog or digital components. A cardiac event sensing thresholdmay be automatically adjusted by sensing circuit 86 under the control ofcontrol circuit 80, based on timing intervals and sensing thresholdvalues determined by control circuit 80, stored in memory 82, and/orcontrolled by hardware, firmware and/or software of control circuit 80and/or sensing circuit 86.

Upon detecting a cardiac electrical event based on a sensing thresholdcrossing, sensing circuit 86 may produce a sensed event signal, such asan R-wave sensed event signal, that is passed to control circuit 80. TheR-wave sensed event signal is used by control circuit 80 for restartinga pacing escape interval timer that controls the basic time intervalsused for scheduling cardiac pacing pulses. For example, in a VVI pacingmode, a ventricular pacing interval (or VV interval) may be restarted inresponse to each R-wave sensed event signal that is received outside ablanking period to inhibit a scheduled pacing pulse. The ventricularpacing interval is started in response to each delivered pacing pulse tocontrol the minimum heart rate and timing of pacing pulses delivered bytherapy delivery circuit 84.

Control circuit 80 may also use the R-wave sensed event signalscorresponding to intrinsic (non-paced) heart depolarizations todetermine RR intervals (RRIs) for detecting tachyarrhythmia anddetermining a need for therapy. An RRI is the time interval between twoconsecutively sensed intrinsic R-waves and may be determined between twoconsecutive R-wave sensed event signals received from sensing circuit86. Control circuit 80 may be configured to detect a tachyarrhythmiabased on RRIs and/or the morphology of QRS waveforms received asmulti-bit digitized signals from sensing circuit 86. Therapy deliverycircuit 84 is controlled to deliver ATP and/or CV/DF shock pulsesaccording to programmed therapy protocols in response to detectingventricular tachycardia or fibrillation.

In this example, therapy delivery circuit 84 includes a high voltage(HV) therapy circuit 83 and may include a low voltage (LV) therapycircuit 85. Each therapy circuit 83 and 85 includes charging circuitry,one or more charge storage devices such as one or more high voltageholding capacitors or low voltage holding capacitors, respectively, andswitching circuitry that controls when the capacitor(s) are charged anddischarged across a selected CV/DF shock vector or pacing electrodevector. Charging of capacitors to a programmed pacing voltage amplitudeand discharging of the capacitors for a programmed pacing pulse widthmay be performed by therapy delivery circuit 84 according to controlsignals received from control circuit 80. For example, a pace timingcircuit included in control circuit 80 may include programmable digitalcounters set by a microprocessor of the control circuit 80 forcontrolling the basic pacing time intervals associated with variouspacing modes or ATP sequences delivered by ICD 14. The microprocessor ofcontrol circuit 80 may also set the amplitude, pulse width, polarity orother characteristics of the cardiac pacing pulses, which may be basedon programmed values stored in memory 82.

Therapy delivery circuit 84 is controlled by control circuit 80 tocharge one or more holding capacitors according to a capacitor chargingmode. As described herein, the therapy delivery circuit 84 may beconfigured to charge one or more holding capacitors according to adelayed charging mode in which capacitor charging is withheld for atleast a portion of or all of a pacing interval. The holding capacitorvoltage may be allowed to fall below the pacing voltage amplitude duringa pacing interval without being recharged. At other times, therapydelivery circuit 84 may be controlled to charge one or more holdingcapacitors according to a charging without delay mode, during whichcapacitor charging is not withheld and may be performed from thebeginning or throughout a pacing interval as needed to maintain theholding capacitor charge at the pacing voltage amplitude. Controlcircuit 80 may be configured to control therapy delivery circuit toswitch between the delayed capacitor charging mode and the capacitorcharging without delay mode based on intrinsic heart rate criteriadetermined from the cardiac electrical signal(s) received by sensingcircuit 86 and/or other signals received from sensor(s) 90.

Components that may be included in HV therapy circuit 83 and LV therapycircuit 85 are described below in conjunction with FIGS. 4 and 5,respectively. It is recognized that the methods disclosed herein forcontrolling capacitor charging for pacing therapy may be implemented inan IMD system that includes only a HV therapy circuit 83 configured todeliver cardiac pacing pulses, which may be in addition to high voltageCV/DF shock delivery capabilities, or in an IMD system that includesonly LV therapy circuit 85 without CV/DF shock therapy capabilities. Insome systems, HV therapy circuit 83 delivers only high voltage CV/DFshock pulses, and LV therapy circuit 85 delivers relatively lowervoltage pacing pulses. In other examples, control circuit 80 mayselectively control which one of HV therapy circuit 83 or LV therapycircuit 85 is utilized for generating and delivering cardiac pacingpulses based on the type of pacing therapy, the pacing threshold voltageamplitude required to capture the heart, or other factors.

ICD 14 may include other sensors 90 for sensing signals from the patientfor use in determining a need for and/or controlling electricalstimulation therapies delivered by therapy delivery circuit 84. In someexamples, a sensor indicative of a need for increased cardiac output maybe included in ICD 14, such as a tissue oxygen sensor, an impedancesensor, or a pressure sensor. A sensor indicative of a need forincreased cardiac output may include a patient activity sensor, such asan accelerometer, or an impedance sensor for determining minute volumeor other respiratory metrics. An increase in the metabolic demand of thepatient due to increased activity may be determined by control circuit80 from a sensor signal received from sensors 90 for use in determininga need for pacing or a need for an increased pacing rate. Likewise, asensor signal may be used by control circuit 80 for determining when aneed for pacing no longer exists or when the pacing rate may bedecreased.

Control circuit 80 may be configured to use a sensor signal from sensors90 to detect a need for pacing and/or an expected increased pacingburden. “Pacing burden” as used herein may be defined as the percentageof time the patient's heart rhythm is a paced rhythm (as opposed to anintrinsic rhythm) over a predetermined period of time. For example, thepatient may be paced 10% of the time over a 24-hour period. In otherexamples, pacing burden may be determined as the proportion of pacedevents to sensed intrinsic events or the proportion of paced events toall paced and sensed intrinsic events combined over a predetermined timeperiod or total number of cardiac events. Control circuit 80 may beconfigured to detect an expected increase in pacing burden based on aphysiological condition of the patient, such as reduced cardiac output,increased patient activity, low tissue oxygenation, or other conditionfor which an increased pacing frequency and/or rate is expected toimprove or alleviate.

The control circuit 80 may respond to detecting an expected change inpacing burden by enabling switching between different capacitor chargingmodes. For example, control circuit 80 may respond to an increase inexpected pacing burden by enabling therapy delivery circuit 84 to switchbetween capacitor charging without delay and delayed capacitor chargingbased on whether increased intrinsic heart rate criteria and/ordecreased intrinsic heart rate criteria are met. Enabling (turning on)the function of switching between capacitor charging modes does notnecessarily require immediately making the switch from one charging modeto another. Rather, after criteria required to be satisfied for enablingthe function of switching between capacitor charging modes, additionalcriteria relating to the intrinsic heart rate and/or sensor signals maybe required to be met before actually performing the switch from onecapacitor charging mode to another.

In other examples, control circuit 80 may respond to detecting a changein expected pacing burden based on a signal from sensors 90 by directlyswitching the capacitor charging mode. For example, when the pacingburden is expected to be decreased, e.g., due to a restored cardiacoutput, tissue oxygenation, blood pressure, or reduced patient activity,the control circuit 80 may control the therapy delivery circuit 84 toswitch the capacitor charging pacing mode to delayed capacitor chargingto conserve energy of power source 98. Methods for enabling the functionof automatic switching between capacitor charging modes and methods forcontrolling the timing of the switching between capacitor charging modesafter the switching function is enabled are described in greater detailbelow in conjunction with the flow charts and timing diagrams presentedherein.

Control parameters utilized by control circuit 80 for sensing cardiacevents, detecting cardiac arrhythmias and controlling therapy delivery,including controlling capacitor charging techniques as disclosed herein,may be programmed into memory 82 via telemetry circuit 88. Telemetrycircuit 88 includes a transceiver and antenna for communicating withexternal device 40 (shown in FIG. 1A) using RF communication or othercommunication protocols as described above. Under the control of controlcircuit 80, telemetry circuit 88 may receive downlink telemetry from andsend uplink telemetry to external device 40. In some cases, telemetrycircuit 88 may be used to transmit and receive communication signalsto/from another medical device implanted in patient 12.

FIG. 4 is schematic diagram 150 of HV therapy circuit 83 included in ICD14 according to one example. HV therapy circuit 83 includes a HVcharging circuit 154 and a HV charge storage and output circuit 160. HVtherapy circuit 83 is shown coupled to a processor and HV therapycontrol circuit 152 which may be included in control circuit 80 forcontrolling HV charging circuit 154 and HV charge storage and outputcircuit 160. HV charge storage and output circuit 160 includes a HVholding capacitor 162 coupled to switching circuitry 166 via a pulsecontrol switch 164 for coupling the HV holding capacitor 162 toelectrodes 24, 26 and/or housing 15 to deliver a desired electricalstimulation pulse, which may be a pacing pulse or a CV/DF shock pulse,to the patient's heart 8. In other examples, pacing and sensingelectrodes 28 and 30 (not shown in FIG. 4) may be selectively coupled toHV holding capacitor 162 via switching circuitry 166 for using one orboth of electrodes 28 and 30 in a pacing electrode vector for deliveringa pacing pulse from HV therapy module 83.

HV holding capacitor 162 is shown schematically as a single capacitor,but it is recognized that a bank of two or more capacitors or otherenergy storage devices may be used to store energy for producingelectrical signals delivered to heart 8. In one example, HV capacitor162 is a series of three capacitors having an effective capacitance of148 microfarads. HV holding capacitor 162 is charged to a desired pacingpulse voltage amplitude (or shock voltage amplitude in the case of aCV/DF shock delivery) by HV charging circuit 154 under the control ofprocessor and HV therapy control 152. It is to be understood thatcharging to a pacing voltage amplitude may include charging to aspecified tolerance within or greater than the pacing voltage amplitude.For example, charging to the pacing voltage amplitude may includecharging to 113% (or another predetermined percentage) of the programmedpacing voltage amplitude.

HV charging circuit 154 receives a voltage regulated signal from powersource 98 (FIG. 3). HV charging circuit 154 includes a transformer 156to step up the battery voltage of power source 98 in order to achievecharging of HV holding capacitor 162 to a voltage that is much greaterthan the battery voltage. Charging of capacitor 162 by HV chargingcircuit 154 is performed under the control of processor and HV therapycontrol 152, which receives feedback signals from HV charge storage andoutput circuit 160 to determine when capacitor 162 is charged to aprogrammed voltage. A charge completion signal is passed to HV chargingcircuit 154 to terminate charging by processor and HV therapy controlmodule 152. One example of a high voltage charging circuit and itsoperation is generally disclosed in U.S. Pat. No. 8,195,291 (Norton, etal.), incorporated herein by reference in its entirety.

When the capacitor charging is being controlled in a capacitor chargingwithout delay mode, charging of HV holding capacitor 162 may occurcontinuously or semi-continuously throughout a pacing interval startedin response to a delivered pacing pulse or sensed intrinsic event.Continuous charging during a pacing interval may be achieved bycomparing the feedback signal from HV charge storage and output circuit160 to the targeted pacing voltage amplitude (plus or minus anytolerance) and performing top-off charging of capacitor 162 as needed tomaintain the capacitor at a desired voltage. For example, the capacitorcharge feedback signal may be compared to the targeted pacing voltageamplitude on every interrupt signal from control circuit 80 or otherpredetermined frequency during a pacing interval. Whenever the charge isbelow the pacing voltage amplitude, the capacitor 162 is charged asneeded to maintain the charge at the programmed pacing voltage amplitudethroughout the pacing interval.

In other examples of a charging without delay mode, the charge ofcapacitor 162 may be compared to the targeted pacing voltage amplitudeat the start of a pacing interval and charged up to the pacing voltageamplitude (plus any specified tolerance) one time during the pacinginterval, without monitoring/charging throughout the pacing interval. Ifa pacing pulse is delivered or leakage of the capacitor charge occursduring the pacing interval after charging, the capacitor charge istopped off at the beginning of the next pacing interval.

As described in conjunction with the timing diagrams and flow chartspresented herein, the processor and HV therapy control 152 may beconfigured to withhold charging of HV holding capacitor 162 whenincreased heart rate criteria and/or decreased pacing burden criteriaare satisfied. The capacitor charging may be delayed or withheld byprocessor and HV therapy control 152 until a pacing interval expires oruntil a capacitor charging delay interval expires.

When HV holding capacitor 162 is charged to a desired pacing voltageamplitude and a pacing interval expires, the HV holding capacitor 162 iscoupled across the desired pacing electrode vector via pulse controlswitch 164 and switching circuitry 166 to deliver the pacing pulse.Switching circuitry 166 may be in the form of an H-bridge and mayinclude switches 180 a-180 c and 182 a-182 c that are controlled bysignals from processor and HV control circuit 152. Switches 180 a-180 cand 182 a-182 c may be implemented as silicon-controlled rectifiers(SCRs), insulated-gate bipolar transistors (IGBTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), and/or otherswitching circuit components. Switches 180 a-180 c and 182 a-182 c arecontrolled to be open or closed by processor and HV therapy controlcircuit 152 at the appropriate times for delivering a monophasic,biphasic or other desired pacing pulse by discharging capacitor 162across the pacing load presented by heart 8 and a selected pacingelectrode vector. The HV holding capacitor 162 is coupled across theselected pacing electrode vector for the programmed pacing pulse widthvia pulse control switch 164.

For instance, the selected electrodes 24, 26 and/or housing 15 may becoupled to HV holding capacitor 162 by opening (i.e., turning off ordisabling) and closing (i.e., turning on or enabling) the appropriateswitches of switching circuitry 166 to pass a desired electrical signalto the therapy delivery electrode vector. The electrical signal may be amonophasic, biphasic or other shaped signal. The signal may be amonophasic or biphasic pacing pulse delivered in response to a pacinginterval expiring such as a VVI pacing interval, a post-shock pacinginterval or an ATP interval. At other times, the signal may be a CV/DFshock for terminating a ventricular tachyarrhythmia when VT or VF isdetected.

To deliver a pacing pulse, for example, one of switches 180 a, 180 b or180 c may be closed simultaneously with one of switches 182 a, 182 b, or182 c without closing both of the “a,” “b” or “c” switches across agiven electrode 24, 26 or housing 15, respectively, at the same time. Todeliver a biphasic pulse using electrode 24 and housing 15, forinstance, switch 180 a and 182 c may be closed to deliver a first phaseof the biphasic pulse. Switches 180 a and 182 c are opened after thefirst phase, and switches 180 c and 182 a are closed to deliver thesecond phase of the biphasic pulse. Switches 180 b and 182 b remain openor disabled in this example with electrode 26 not selected or used inthe therapy delivery vector. In other examples, electrode 26 may beincluded instead of electrode 24 or simultaneously activated withelectrode 24.

After delivering of a pacing pulse, the HV holding capacitor 162 may bere-charged to the programmed pacing pulse amplitude if increasedintrinsic heart rate detection criteria are not satisfied. Processor andHV therapy control 152, however, may withhold charging of HV holdingcapacitor 162 to the pacing voltage amplitude in response to determiningthat increased intrinsic heart rate criteria are satisfied using thetechniques disclosed herein. When the rate of intrinsic events sensed bysensing circuit 86 of ICD 14, the slope of the rate of intrinsic events,and/or other criteria satisfy increased intrinsic heart rate detectioncriteria and/or decreased pacing burden criteria, charging of HV holdingcapacitor 162 may be withheld until a pacing interval expires or until apredetermined charging delay interval expires. Processor and HV therapycontrol circuit 152 may revert to charging HV holding capacitor 162without delay, e.g., at the beginning of a pacing interval or throughouta pacing interval started in response to a pacing pulse or a sensedevent signal as needed, in response to determining that decreasedintrinsic heart rate criteria and/or decreased pacing burden criteriaare met as described below.

FIG. 5 is a conceptual diagram of LV therapy circuit 85 according to oneexample. LV therapy circuit 85 may include a LV charging circuit 330, acapacitor selection and control circuit 332, and a capacitor array 350.Capacitor array 350 may include multiple LV holding capacitors 352, 354,356 and 358 that can each be charged by LV charging circuit 350 to aprogrammed pacing voltage amplitude. The LV holding capacitors 352, 354,356 and 358 are coupled to a respective output capacitor 372 a-372 d(collectively 372), 376, or 378 via respective switches 362, 364, 366,and 368 to deliver pacing pulses. Each of LV holding capacitors 352,354, 356 and 358 may have a capacitance that is less than the effectivecapacitance of HV holding capacitor 162 of HV therapy circuit 83. Forexample, each of holding capacitors 352, 354, 356 and 358 may have acapacitance of up to 6 microfarads, up to 10 microfarads, up to 20microfarads or other selected capacitance, but all may have acapacitance significantly less than the effective capacitance of HVholding capacitor 162 and have a lower voltage rating than HV holdingcapacitor 162.

Power source 98 (FIG. 3) may provide regulated power to LV chargingcircuit 330. LV charging circuit 330 may be controlled by a statemachine in capacitor selection and control circuit 332 to charge all orselected LV holding capacitors 352, 354, 356 and 358 using a multiple ofthe battery voltage of power source 98, e.g., four times the batteryvoltage. LV charging circuit 330 charges one or more of capacitors 352,354, 356 and 358 as needed for delivering a pacing pulse to thepatient's heart via a selected pacing electrode vector. The pacing pulsemay be a single pacing pulse delivered by discharging a single LVholding capacitor for a programmed pulse width. In other examples, twoor more of LV holding capacitors 352, 354, 356 and 358 may be dischargedsequentially to deliver two or more fused pulses within a pacing pulsewidth to deliver a composite pacing pulse.

In some examples, the LV therapy circuit 85 includes three pacingchannels 342, 344 and 346. Each pacing channel is capable of producing asingle pacing pulse when a respective LV holding capacitor 352, 356 or358 is discharged across an output capacitor 372, 376, or 378,respectively. Pacing channel 342 includes a back-up holding capacitor354 that may be used for delivering back-up pacing pulses. Back-upholding capacitor 354 may be used to deliver an individual pulse of acomposite pacing pulse. Depending on the number of extra-cardiovascularelectrodes coupled to ICD 14, one or more channels may include multipleselectable output signal lines. For example, channel 342 is shown inthis example to include multiple selectable pacing output signal lines382 a-382 d that may be selectively coupled to LV holding capacitor 352and back-up holding capacitor 354 via closure of one or more ofelectrode selection switches 374 a-374 d. For example, multipleelectrodes carried by lead 16 may be coupled to pacing channel 342, anda pacing electrode vector may be selected from the multiple electrodesby closing certain ones of switches 374 a-374 d.

Pacing channels 344 and 346 are shown having single output signal lines386 and 388 that are coupled to respective LV holding capacitors 366 and368 via respective switches 366 and 368. In other examples, all threepacing channels 342, 344 and 346 may be provided with a single outputsignal line or with multiple output signal lines to enable selection ofa pacing electrode vector from among multiple extra-cardiovascularelectrodes coupled to ICD 14, e.g., any of electrodes 24, 26, 28, or 30of lead 16 shown in FIG. 1A.

When a pacing therapy is needed, control circuit 80 may control LVtherapy circuit 85 to select any one or combination of the pacingchannels 342, 344 and 346 to deliver a pacing pulse. The pacing pulsemay be a single-pulse pacing pulse delivered by discharging one of theholding capacitors 352, 354, 356 or 358 across a selected pacingelectrode vector via a respective output capacitor 374, 376 or 378 whena respective switch 362, 364, 366 or 368 is closed. The output line 382a, 382 b, 382 c, or 382 d used to deliver pacing current from pacingchannel 342 may be selected via a respective electrode selection switch372 a-372 d. The switch 362, 364, 366 or 368 that enables discharge of aholding capacitor 352, 354, 356 or 358, respectively, may be enabled bycapacitor selection and control circuit 332 at the appropriate time whena pacing pulse is needed and maintained in an active, enabled (closed)state until the single-pulse pacing pulse width is expired.

In some patients, a single-pulse pacing pulse generated by LV therapycircuit 85 may not have the pulse energy required to capture thepatient's heart. Control circuit 80 may control LV therapy circuit 85 todeliver fused pulses in a multi-pulse composite, pacing pulse. Two orall three pacing channels 342, 344 and 346 are tied together by switches360 a-d and 370 to enable individual pulses to be delivered across aselected pacing electrode vector from a single output signal line 344.For example, control circuit 80 may control LV therapy circuit 85 todeliver a multi-pulse, composite pacing pulse by activating at least oneof switches 360 a-d and switch 370 to tie at least one of pacing outputlines 382 a-d and pacing output line 388 to pacing channel 344. Controlcircuit 80 controls capacitor selection and control circuit 332 toenable pacing channel switches 362, 364, 366 and 368 (and at least oneelectrode selection switch 372 a-d of pacing channel 342) in asequential manner to sequentially couple two or more of the respectiveholding capacitors 352, 354, 356 or 358 to output signal line 386 todeliver a sequence of at least two fused, individual pulses to produce acomposite pacing pulse.

In various examples, depending on the particular pacing channel and leadand electrode configuration used with ICD 14, some electrode selectionswitches shown in FIG. 5 may not be required. Furthermore, it isrecognized that less than four holding capacitors or more than fourholding capacitors may be included in a capacitor array 350 for use indelivering a sequence of fused pulses in a composite pacing pulse whenthe LV therapy circuit 85 is controlled to deliver a pacing pulse.

Capacitor selection and control circuit 332 selects which holdingcapacitors 352, 354, 356 or 358 are coupled to output line 386 and inwhat sequence by controlling respective switches 362, 364, 366 and 368.A sequence of pulses may be delivered to produce a composite pacingpulse by sequentially discharging holding capacitors 352, 354, 356 or358 one at a time (or one combination at a time) across a respectiveoutput capacitor 372, 376 or 378 by sequentially enabling or closing therespective switches 362, 364, 366 or 368. For example, at least two ofholding capacitors 352, 354, 356 or 358 are sequentially discharged toproduce a composite pacing pulse produced by at least two fusedindividual pulses. Output line 386 may be electrically coupled to apacing cathode electrode carried by lead 16 and a return anode electrodecarried by lead 16 (or housing 15) may be coupled to ground. The pacingcathode electrode and return anode electrode may correspond toelectrodes 28 and 24, respectively, as shown in FIG. 1A in one example,however any pacing electrode vector may be selected from electrodes 24,26, 28, and 30 and/or housing 15 shown in FIG. 1A.

In some examples, a low-voltage, fused pacing pulse is delivered bydelivering an individual pulse from pacing channel 344 and 346sequentially followed by a third, longer individual pulse delivered bypacing channel 342 by discharging both capacitors 352 and 354simultaneously. The first two individual pulses may be 2.0 ms in pulsewidth and the third pulse may be 4.0 ms in pulse width for a compositepacing pulse width of 8 ms. The higher capacitance of the parallelcapacitors 312 and 314 allows for the third individual pulse to belonger in pulse width while maintaining a pulse amplitude thatsuccessfully captures the heart. All three individual pulses aredelivered via output line 386 by controlling output configurationswitches 360 and 370 to couple the capacitors 352, 354 and 358 to outputline 386. Other examples of a LV therapy circuit and pacing pulsegeneration techniques that may be used in conjunction with thetechniques disclosed herein are generally disclosed in U.S. patentapplication Ser. No. 62/262,412 (Attorney Docket No. C00012192.USP1) andthe corresponding US. Pat. application Ser. No. 15/368,197 (AttorneyDocket No. C00012192.USU2).

The LV holding capacitors 352, 354, 356 and/or 358 selected for use ingenerating a single-pulse pacing pulse or a multi-pulse, compositepacing pulse may be charged by LV charging circuit 330 according to thecapacitor charging control techniques disclosed herein. When criteriafor detecting an increased intrinsic heart rate are satisfied, chargingof LV holding capacitors 352, 354, 356 and/or 358 used for generatingpacing pulses may be withheld or delayed according to a delayedcapacitor charging pacing mode. Charging is delayed until a pacinginterval expires in some examples or until a capacitor charging delayinterval expires in other examples. If criteria for detecting adecreased intrinsic heart rate are satisfied, charging of the LV holdingcapacitors 352, 354, 356 or 358 used for generating and deliveringpacing pulses is performed without delay, e.g., at the beginning of apacing interval or throughout the pacing interval as generally describedabove in conjunction with FIG. 4.

Control circuit 80 may enable the function of automatic switchingbetween capacitor charging modes based on pacing burden criteria in someexamples, as described in further detail herein. After automaticswitching between charging modes is enabled based on a change in theactual or expected pacing burden, control circuit 80 may switch betweencontrolling LV charging circuit 330 according to a delayed charging modeand a charging without delay mode. The switching between the chargingmodes may be controlled based on intrinsic heart rate criteria and/orpacing burden criteria.

FIG. 6 is a flow chart 100 of one method for controlling holdingcapacitor charging for pacing pulse delivery. At block 101, controlcircuit 80 enables automatic switching between a first capacitorcharging mode and a second capacitor charging mode. In some examples,charging mode switching is enabled in response to a user commandreceived from external device 40 via telemetry circuit 88. In otherexamples, control circuit 80 may automatically enable and/or disablecharging mode switching based on determining an actual or expectedchange in pacing burden, e.g., as described in conjunction with FIG. 12below. The two charging modes may include a delayed charging mode and acharging without delay mode.

In the example shown in FIG. 6, ICD 14 may initially be operating in acapacitor charging without delay mode. In this charging mode, one ormore holding capacitors are charged to the pacing voltage amplitudeduring each pacing interval according to a selected pacing outputconfiguration, e.g., using either HV therapy circuit 83 or LV therapycircuit 85 as described above. The pacing interval is set by controlcircuit 80 in response to a delivered pacing pulse or sensed cardiacevent, e.g., an intrinsic R-wave, to control the timing of pacingpulses, e.g., according to a VVI or other pacing mode. The holdingcapacitor charging is performed without delay such that charging maybegin at the start of the pacing interval and/or occur at any timeduring the pacing interval in response to a comparison of the holdingcapacitor charge to the programmed pacing voltage. If the holdingcapacitor charge is less than the programmed pacing voltage amplitude(or less than a tolerance below the pacing voltage amplitude), controlcircuit 80 controls therapy delivery circuit 84 to charge the holdingcapacitor to the programmed pacing voltage amplitude.

While operating in the capacitor charging without delay mode, controlcircuit 80 monitors the cardiac electrical signal received by sensingcircuit 86 for determining if increased intrinsic heart rate criteriaare met at block 104. Increased intrinsic heart rate criteria mayrequire that the intrinsic heart rate be equal to or greater than apredetermined mode switching heart rate threshold. The mode switchingheart rate threshold may be defined to be faster than the current pacingrate such that mode switching does not necessarily occur in response toone or more sensed events occurring at event intervals shorter than thepacing interval and resulting in one or more inhibited pacing pulses.For instance, charging mode switching may not occur if the heart rate isbetween the pacing rate and the mode switching heart rate threshold.Various techniques for determining if increased intrinsic heart ratecriteria are met are described below, e.g., in conjunction with FIGS. 7and 12.

If the increased intrinsic rate criteria are satisfied at block 104,control circuit 80 switches to the second charging mode, the delayedcapacitor charging mode in this example, at block 106. In this mode,control circuit 80 may withhold comparisons between the holdingcapacitor charge and the programmed pacing voltage amplitude and/orwithhold charging of the holding capacitor(s) even when the capacitorcharge is less than the pacing voltage amplitude. Capacitor charging iswithheld for at least a portion of the pacing interval started inresponse to a cardiac event, either a delivered pacing pulse or a sensedintrinsic event. Charging may be withheld for the entire pacing intervalin some examples. In other examples, charging is withheld untilexpiration of a charging delay interval. If an intrinsic event is sensedprior to expiration of the charging delay interval, no charging occursand the pacing interval is restarted.

During the delayed capacitor charging mode, control circuit 80 monitorsthe cardiac electrical signal for determining whether decreasedintrinsic heart rate criteria are met at block 108. The decreasedintrinsic heart rate criteria may be satisfied in response to one ormore pacing intervals expiring and/or one or more charging delayintervals expiring. As such, in some examples decreased intrinsic heartrate criteria may be satisfied in response to a decreasing intrinsicheart rate before the intrinsic heart rate falls below the pacing rate,before a pacing interval expires. In response to the decreased intrinsicheart rate criteria being met, control circuit 80 switches back to thecapacitor charging without delay mode at block 102. Methods fordetermining if decreased intrinsic heart rate criteria are described ingreater detail below, e.g., in conjunction with FIGS. 7, 8A-D, and 12.

In this way, capacitor charging is controlled according to a delayedcapacitor charging mode after increased intrinsic heart rate criteriaare met and the potential need for a pacing pulse is expected to be low.Capacitor charging is controlled according a charging without delay modeafter decreased intrinsic heart rate criteria are met and the potentialneed for a pacing pulse is relatively higher. In some examples, controlcircuit 80 may monitor one or more signals from sensors 90 in additionto or alternatively to the intrinsic cardiac electrical events sensed bysensing circuit 86 for determining if increased intrinsic heart ratecriteria and/or decreased intrinsic heart rate criteria are met atblocks 104 and 108, respectively.

FIG. 7 is a flow chart 110 of a method for controlling holding capacitorcharging based on intrinsic heart rate criteria according to oneexample. The method of FIG. 7 may be implemented for delivering a pacingtherapy by either HV therapy circuit 83 or LV therapy circuit 85. Themethod of flow chart 110 controls the timing of holding capacitorcharging, which may be HV holding capacitor 162 of HV therapy circuit 83or one or more of the LV holding capacitors 352, 354, 356, or 358 of LVtherapy circuit 85 depending on which of HV therapy circuit 83 or LVtherapy circuit 85 is selected for delivering the pacing therapy.

At block 112, a pacing pulse is delivered by the selected HV therapycircuit 83 or LV therapy circuit 85. The pacing pulse is delivered uponexpiration of a pacing interval, which may be a ventricular pacinginterval during VVI pacing or another pacing interval according toanother pacing therapy or pacing mode. Control circuit 80 restarts thepacing interval at block 114 in response to delivery of the pacingpulse. Control circuit 80 controls the charging circuitry of theselected HV therapy circuit 83 or LV therapy circuit 85 to charge theholding capacitor(s) back up to the pacing voltage amplitude at block115 during the pacing interval without delay. In this way, if the pacinginterval expires without a sensed event, the therapy circuit 83 or 85 isprepared to deliver the next pacing pulse upon pacing intervalexpiration.

At block 116, the control circuit 80 waits for the pacing interval toexpire. If the pacing interval expires without sensing circuit 86sensing an intrinsic cardiac event, e.g., an R-wave, at block 116, thescheduled pacing pulse is delivered in response to the expired pacinginterval at block 112. Control circuit 80 restarts the pacing intervalat block 114.

If sensing circuit 86 does sense an intrinsic cardiac event during thepacing interval at block 118, a sensed event signal may be passed fromsensing circuit 86 to control circuit 80. In response to receiving thesensed event signal, e.g., an R-wave sensed event signal, from sensingcircuit 86 before the pacing interval expires (“yes” branch of block116), control circuit 80 inhibits the scheduled pacing pulse byre-starting the pacing interval at block 118.

Control circuit 80 may be configured to monitor for an increasedintrinsic heart rate that is a predetermined rate greater than theprogrammed pacing rate for controlling capacitor charging modeswitching. In one example, control circuit 80 may detect an increase inthe intrinsic heart rate by starting a hysteresis interval at block 118at the same time that the pacing interval is started. The hysteresisinterval may be set equal to or shorter than the pacing interval. Forinstance, the hysteresis interval may be at least 10 to 30 ms shorterthan the pacing interval. In this way, detection of an increasedintrinsic heart rate for switching to a delayed capacitor charging modemay require a higher intrinsic heart rate than the intrinsic heart raterequired to withhold pacing.

In one example the hysteresis interval is 15 ms shorter than the pacinginterval when the pacing interval is 1 second, corresponding to a pacingrate of 60 pulses per minute. The hysteresis interval of approximately0.985 ms corresponds to a heart rate of 65 beats per minute, about 5pulses per minute faster than the pacing rate. The hysteresis intervalmay be determined by control circuit 80 as a fixed interval less thanthe pacing interval currently in effect. In other examples, controlcircuit 80 may determine the hysteresis interval by determining a sensedevent interval corresponding to an intrinsic heart rate that is a fixedrate less than the pacing rate currently in effect, e.g., 5 to 15 beatsless than the current pacing rate. In other examples, the hysteresisinterval may be the same as the pacing interval.

Control circuit 80 may control the therapy delivery circuit 84 tomaintain the charge of the holding capacitor at the programmed pacingvoltage amplitude at block 112 after inhibiting the scheduled pacingpulse. The capacitor charge may be monitored during the pacing intervalset in response to a sensed event, and if the charge drops more than avoltage tolerance below the pacing voltage amplitude, the charge of theholding capacitor may be topped off back to the pacing voltageamplitude.

Different protocols or techniques may be used to control topping off ormaintaining the capacitor charge during pacing intervals. In oneexample, when LV therapy circuit 85 is used to deliver pacing pulses,one or more LV holding capacitors may be charged to the pacing voltageamplitude at the start of each pacing interval. In some cases, chargingto the pacing voltage amplitude is controlled by charging to the pacingvoltage amplitude plus a tolerance, e.g., to 110% to 115% of theprogrammed pacing voltage amplitude. In one example, the LV holdingcapacitor(s) used for delivering a pacing pulse are charged to 113% ofthe programmed pacing voltage amplitude at the start of each pacinginterval that is reset in response to a delivered pacing pulse or sensedintrinsic event.

In other examples, for instance if the HV therapy circuit 83 iscontrolled to deliver pacing pulses, the HV holding capacitor 162 ischarged to the pacing voltage amplitude (or the pacing voltage amplitudeplus a tolerance), and continuous top-off charging is performed at block120 until a pacing interval expires and a pacing pulse is delivered. Inthis example, processor and HV therapy control 152 may receive acapacitor charge signal from HV therapy circuit 83 indicating thevoltage across HV holding capacitor 162, e.g., on each interrupt clocksignal. Processor and HV therapy control 152 may compare the capacitorcharge signal to a capacitor charge threshold and control HV chargingcircuit 154 to perform top-off charging following any interrupt clocksignal during an unexpired pacing interval as needed to maintain the HVholding capacitor charge at the pacing voltage amplitude (plus or minusa specified tolerance). The specific protocol used to maintain a holdingcapacitor at the pacing voltage amplitude in a ready state for pacingpulse delivery may vary between devices but generally includes top-offcharging to the pacing voltage amplitude (or the pacing voltageamplitude plus a tolerance) during each pacing interval as needed tomaintain the holding capacitor charge at the pacing voltage amplitude.

If sensing circuit 86 does not sense an intrinsic event during thepacing interval started at block 118, “no” branch of block 122, controlcircuit 80 controls the therapy delivery circuit 84 to deliver thescheduled pacing pulse at block 112 in response to the expiration of thepacing interval. Control circuit 80 continues to charge the holdingcapacitor(s) according to the charging without delay mode during eachpacing interval set in response to each delivered pacing pulse andsensed cardiac event.

If an intrinsic event is sensed by the sensing circuit 86 during thepacing interval started at block 118 (“yes” branch of block 122),control circuit 80 determines at block 124 if the sensed event occurredbefore the hysteresis interval expired. If the sensed event occurredafter the hysteresis interval expired but before the pacing intervalexpired (“no” branch of block 124), the control circuit 80 inhibits thescheduled pacing pulse by restarting the pacing and hysteresis intervalsat block 118. The holding capacitor charge is maintained at the pacingvoltage amplitude at block 120 according the charging without delaymode.

If an intrinsic event is sensed by the sensing circuit 86 before thehysteresis interval expires (“yes” branch of block 124), control circuit80 may increase the value of a counter at block 125. The counter may bepreviously initialized to zero and is used to count the number ofcardiac cycles during which an intrinsic event is sensed prior to theexpiration of the hysteresis interval. Control circuit 80 may beconfigured to detect an increased intrinsic heart rate based on athreshold number of cardiac cycles having a sensed event occurringduring the hysteresis interval. If the counter has not reached thethreshold number of cardiac cycles for detecting an increased intrinsicheart rate, “no” branch of block 126, control circuit 80 restarts thepacing and hysteresis intervals at block 118 and continues to maintainthe charge of the holding capacitor at the pacing voltage amplitude in aready state for pacing pulse delivery, according to the charging withoutdelay mode.

If the counter reaches the threshold number of cardiac cycles having asensed event during the hysteresis interval, as determined at block 126,control circuit 80 detects an increased intrinsic heart rate that isequal to or greater than the rate corresponding to the hysteresisinterval. The threshold number of cardiac cycles having an intrinsicevent sensed within the hysteresis interval required to detect anincreased intrinsic heart rate may be one or more. In some examples, anintrinsic event may be required to be sensed within the hysteresisinterval for at least five cardiac cycles in order to detect anincreased intrinsic heart rate. The threshold number of cardiac cyclesmay be required to be consecutive in some examples, e.g., at least threeconsecutively sensed intrinsic events at or above the hysteresis ratecorresponding to the hysteresis interval.

In other examples, the control circuit 80 may include an X of Y countersuch that the cardiac cycles having intrinsic events sensed withinrespective hysteresis intervals may not be required to be consecutive,e.g., three out of five cardiac cycles, four out of six cardiac cycles,eight out of ten cardiac cycles or other ratio or percentage. In someexamples, all Y cardiac cycles may be required to be sensed cardiaccycles with no paced cardiac cycles. For example, if four out of sixcardiac cycles are required to include intrinsic events sensed withinthe hysteresis interval, the other two cardiac cycles may be required toinclude sensed intrinsic events within the pacing interval. None of thesix cardiac cycles are paced cardiac cycles. In other examples, the Ycardiac cycles may include both paced and sensed cardiac cycles but atleast X cardiac cycles are required to include a sensed intrinsic eventduring the hysteresis interval in order for an increased intrinsic heartrate to be detected.

At block 127, control circuit 80 determines that increased intrinsicheart rate criteria are met in response to the counter reaching thethreshold number of cardiac cycles that each include an intrinsic sensedevent during the respective hysteresis interval. In response to sensingthe latest intrinsic event within the hysteresis interval that causesthe increased intrinsic heart rate to be detected, the next scheduledpacing pulse is inhibited by restarting the pacing interval at block 128without delivering a pacing pulse. Control circuit 80 switches to adelayed capacitor charging mode by not starting the hysteresis intervalat block 128 and withholding capacitor charging at block 129. Theholding capacitor charge is not maintained at the pacing voltageamplitude during the pacing interval after the heart rate has reached orexceeded the rate corresponding to the hysteresis interval for thethreshold number of cardiac cycles.

If an intrinsic event is sensed by the sensing circuit 86 during thepacing interval started at block 128, as determined at block 130,control circuit 80 inhibits the scheduled pacing pulse by restarting thepacing interval at block 128 and continues to withhold capacitorcharging at block 129. If the pacing interval expires at block 130without an intrinsic event being sensed by the sensing circuit 86, thecounter used to count the number of events within the hysteresisinterval may be reset to zero at block 132. In response to the pacinginterval expiring at block 130 without a sensed intrinsic event, controlcircuit 80 controls therapy delivery circuit 84 to charge the holdingcapacitor at block 130 and deliver the scheduled pacing pulse at block112 as soon as the selected holding capacitor reaches the programmedpacing voltage amplitude. The time required to charge the holdingcapacitor(s) at block 136, after the pacing interval has expired, maydelay the delivery of the pacing pulse at block 112. After the pacingpulse is delivered, the pacing interval is restarted at block 114, andthe control circuit 80 controls the therapy delivery circuit 84 tocharge the holding capacitor(s) during the pacing interval at block 115according to the charging without delay mode.

In the example shown, expiration of the pacing interval a single time atblock 130 during delayed capacitor charging mode may cause the controlcircuit 80 to return to the charging without delay mode for controllingcharging of the holding capacitor(s) beginning at the start of thepacing interval after each pacing pulse or sensed event and maintainingthe holding capacitor charge at the pacing voltage amplitude during thepacing interval as needed. In other examples, more than one pacing pulsemay be required to be delivered due to an expired pacing interval duringthe delayed charging mode before reverting back to charging the holdingcapacitor without delay after each delivered pacing pulse. As a result,more than one pacing pulse may be delivered at a delayed time intervalafter expiration of the pacing interval due to the time required forcharging the holding capacitor to the pacing voltage amplitude afterexpiration of the pacing interval.

After returning to block 112, maintaining the holding capacitor in a“ready” state by charging to the pacing voltage amplitude and toppingoff the charge as needed until a pacing interval expires may continueuntil an increased intrinsic heart rate is detected again. The increasedintrinsic heart rate is detected according to predetermined criteriawhich may include a hysteresis interval and a required number of cardiaccycles having an intrinsic event sensed within the hysteresis interval,where the hysteresis interval may be shorter than the pacing interval.

FIGS. 8A through 9C are timing diagrams depicting operations performedby ICD 14 in controlling holding capacitor charging based on the timingof sensed intrinsic events. FIGS. 8A and 8B are timing diagramsdepicting operations performed by ICD 14 during the capacitor chargingwithout delay mode. In FIG. 8A, timing diagram 200 shows two pacingpulses 201 and 203 delivered by ICD 14 separated in time by a pacinginterval 205. During pacing, control circuit 80 may control therapydelivery circuit 84 to charge a holding capacitor during each pacinginterval, until increased intrinsic heart rate criteria are met, whichmay be based on a hysteresis interval 206 shorter than the pacinginterval 205.

Upon delivering pacing pulse 201, therapy delivery circuit 84 may becontrolled to charge the holding capacitor(s) used for delivering pacingpulse 201 during a capacitor charging time 204, according to thecharging without delay mode. As described above, the capacitor chargedduring charging time 204 may be the HV holding capacitor 162 (FIG. 4) orany combination of LV holding capacitors 352, 354, 356, and/or 358 (FIG.5).

The charging time 204 is not necessarily a fixed time interval. Ratherthe charging time 204 is the time required to recharge the holdingcapacitor(s) back to the pacing voltage amplitude after pacing pulse 201is delivered and will depend on the pacing voltage amplitude, theresidual charge left on the holding capacitor after pacing pulsedelivery, the capacitance of the holding capacitor, and other factors.While charging time 204 is shown as a single discrete time interval atthe beginning of pacing interval 206, it is recognized that thecapacitor charge may be monitored throughout pacing interval 205 andtopped off as needed if the capacitor charge decreases below the pacingvoltage amplitude due to leakage current in the ICD circuitry. Thecharging time 204 represented as a block of time is intended torepresent the capacitor charging without delay mode, which may includecharging at the beginning and/or throughout the pacing interval asneeded to recharge the holding capacitor to the pacing voltage amplitudeduring the pacing interval.

Pacing interval 205, which may be a VV interval during VVI pacing forexample, is started upon delivery of the first pacing pulse 201. If nointrinsic events are sensed during the pacing interval 205, pacing pulse203 is delivered by therapy delivery circuit 84 in response to theexpiration of pacing interval 205. The holding capacitor is rechargedduring charging time 208 following pacing pulse 203 according to thecharging without delay mode.

FIG. 8B is a timing diagram 210 showing inhibition of a pacing pulse inresponse to a sensed intrinsic event during the pacing interval 205.Pacing pulse 211 is delivered, and the holding capacitor is recharged tothe pacing voltage amplitude during pacing interval 205 without delay,as indicated capacitor charging time 214. If an intrinsic event issensed during the pacing interval 205, the scheduled pacing pulse 213 iswithheld (as indicated by dashed line). Sensing circuit 86 may beconfigured to produce a sensed event signal 216, e.g., an R-wave sensedevent signal, that is passed to control circuit 80. In response to thesensed event signal 216, the pacing interval 205 is restarted as newpacing interval 215, inhibiting the scheduled pacing pulse 213.

The control circuit 80 may monitor the charge of the holding capacitorand top off the capacitor charge during recharging time 218 if needed tomaintain the holding capacitor voltage at the pacing voltage amplitude(or within a specified tolerance voltage of the pacing voltageamplitude) during the pacing interval 215. The sensed event signal 216that is within the pacing interval 205 but not within the hysteresisinterval 206 does not alter the control of capacitor charging in thisexample. The holding capacitor continues to be recharged during eachpacing interval 205 (or 215) as needed according to the charging withoutdelay mode in order to prepare and maintain the holding capacitor at thepacing voltage amplitude. The holding capacitor may be charged duringpacing interval 205 following a pacing pulse 211 as well as during apacing interval 215 following a sensed event signal 216 if charging isneeded to top off the capacitor charge to the pacing voltage amplitude.Even though a pacing pulse has not been delivered, the holding capacitorcharge may fall below the pacing voltage amplitude due to inherentleakage current in the ICD circuitry. Top off capacitor charging may notbe required immediately following every sensed event. Control circuit 80may be configured to monitor the holding capacitor voltage at the startof each pacing interval and/or throughout each pacing interval, startcharging if the voltage is less than a tolerance below the pacingvoltage amplitude, and terminate charging when the charge is back up tothe pacing voltage amplitude.

If the intrinsic heart rate is less than a hysteresis rate correspondingto hysteresis interval 206 but greater than the pacing ratecorresponding to pacing interval 205, the holding capacitor chargecontinues to be monitored and maintained at the pacing voltage amplitudeaccording to the charging without delay mode. As such, if the sensedevent signal 216 occurs after expiration of the hysteresis interval 206,capacitor charging is performed as needed without delay.

FIGS. 8C and 8D are timing diagrams depicting operations of ICD 14performed for switching from the charging without delay mode to thedelayed charging mode in response to increased intrinsic heart ratecriteria being met. FIG. 8C is a timing diagram showing pacing pulse 221followed by capacitor charge time 214 according to the charging withoutdelay mode. The pacing interval 205 and the hysteresis interval 206 arestarted simultaneously in response to the delivered pacing pulse 221.The next scheduled pacing pulses 223 is inhibited due to a sensedintrinsic event signal 226 that is received from sensing circuit 86 bycontrol circuit 80 during pacing interval 205. The pacing interval 205is restarted as new pacing interval 225 in response to sensed eventsignal 226.

During the charging without delay mode, control circuit 80 may monitorfor an increase in heart rate, faster than the pacing rate correspondingto pacing interval 205, by counting the number of cardiac cycles havingan intrinsic event sensed during the hysteresis interval 206. In theexample shown, control circuit 80 detects an increased intrinsic heartrate in response to the sensed event signal 226 occurring withinhysteresis interval 206. Control circuit 80 switches from the chargingwithout delay mode to the delayed charging mode in response to detectingthe increased intrinsic heart rate by withholding capacitor chargingduring the next pacing interval 225.

This withholding of capacitor charging is illustrated schematically bydashed capacitor charging time block 224, during which no capacitorcharging actually occurs. If the sensed event signal 226 had occurredafter hysteresis interval 206 as described in conjunction with FIG. 8B,capacitor charging time 224 would occur during pacing interval 225 asneeded to maintain the holding capacitor charge at the pacing voltageamplitude. Capacitor charging may be withheld during pacing interval 225by withholding monitoring of the capacitor charge during the pacinginterval 225 or by withholding charging even when the capacitor chargefalls below the pacing voltage amplitude. In the example of FIG. 8C, asensed event signal 226 during a single hysteresis interval 206 is shownto cause control circuit 80 to switch from charging without delay to thedelayed capacitor charging mode and withhold capacitor charging. Inother examples, a sensed event signal during each of more than onerespective hysteresis interval as described above in conjunction withFIG. 7 may be required for increased intrinsic heart rate criteria to bemet to cause switching to the delayed charging mode. For instance,sensed event signal 226 may be the Xth event sensed within a respectivenumber of X hysteresis intervals out of Y cardiac cycles, causing theincreased intrinsic heart rate criteria to be met.

FIG. 8D is a timing diagram 230 depicting operations performed by ICD 14in controlling holding capacitor charging in response to detecting anincreased heart rate based on hysteresis interval 206 according toanother example. In the example of FIG. 8D, three consecutive sensedevent signals 226, 227 and 228 are required to each be sensed within arespective hysteresis interval 206, 207 and 209 in order to detect anincreased intrinsic heart rate and switch the capacitor charging mode tothe delayed charging mode. Each hysteresis interval 206, 207 and 209 isstarted in response to the preceding cardiac event, pacing pulse 221,sensed event signal 226 and sensed event signal 227, respectively. Inthis example, criteria for detecting an increased intrinsic heart rateis not met until the three consecutively sensed events 226, 227 and 228are sensed within the hysteresis interval 206, 207 and 208 set for therespective cardiac cycles. As such, control circuit 80 does not withholdor delay capacitor charging until the increased heart rate detectioncriteria are satisfied. Control circuit 80 controls the chargingcircuitry of therapy delivery circuit 84 to charge the holdingcapacitor(s) used for generating pacing pulses to the pacing voltageamplitude as needed during each of pacing intervals 225 and 229 startedin response to sensed event signals 226 and 227 even though the sensedevent signals 226 and 227 each occurred above the hysteresis rate,within respective hysteresis intervals 206 and 207.

Recharging of the holding capacitor(s) may occur during charging times232 and 234 at the beginning of the respective pacing intervals 225 and229 or throughout the pacing intervals 225 and 229 as needed to top-offthe holding capacitor charge within a tolerance of the pacing voltageamplitude. It is recognized, that depending on the duration of thepacing interval, the inherent leakage current and other factors,recharging of the holding capacitor(s) may not be required during everypacing interval in order to maintain the capacitor charge within aspecified tolerance of the pacing voltage amplitude. However, controlcircuit 80 may monitor or check the charge of the holding capacitorduring each pacing interval 225 and 229 that is started in response tothe sensed event signals 226 and 227, respectively, until criteria fordetecting an increase in heart rate are satisfied.

Upon detecting the third sensed event signal 228 within hysteresisinterval 209, an increased intrinsic heart rate that is faster than orequal to the hysteresis rate is detected in this example. In response,control circuit 80 switches to the delayed charging mode by withholdingcapacitor charging during the next pacing interval 231. The HV or LVcharging circuit of therapy delivery circuit 84 being used for pacingpulse generation may be turned “off” by withholding power supplied tothe charging circuit for charging the holding capacitor. Withholding ofcapacitor charging is illustrated by the dashed box 236. No chargingoccurs following sensed event signal 228 during the first pacinginterval 231 after increased intrinsic heart rate criteria are met.Monitoring of the holding capacitor voltage, e .g, by a comparatorcomparing the capacitor voltage to the programmed pacing voltageamplitude, that may normally be performed to control top-off charging ofthe holding capacitor during a pacing interval may be disabled inresponse to detecting the increased heart rate since no charging of theholding capacitor is performed again until at least one pacing intervalexpires or until other decreased intrinsic heart rate criteria aresubsequently satisfied.

FIGS. 9A-9C are timing diagrams depicting operations performed by ICD 14for withholding capacitor charging according to a delayed capacitorcharging mode and switching back to the charging without delay mode inresponse to decreased intrinsic heart rate criteria being met. Holdingcapacitor charging is withheld after increased intrinsic heart ratecriteria are met as described above in conjunction with FIGS. 8C and 8D.As shown in FIG. 9A, the pacing interval 245 is started in response to asensed event signal 242. No capacitor charging is performed duringpacing interval 245 because capacitor charging is being withheldaccording to the delayed charging mode due to increased intrinsic heartrate criteria being previously met. Pacing interval 245 expires withouta sensed intrinsic event. Control circuit 80 controls therapy deliverycircuit 84 to charge the holding capacitor(s) in response to the pacinginterval 245 expiring as indicated by charging time 244. Once theholding capacitor voltage has reached the pacing voltage amplitude,pacing pulse 243 is delivered. Pacing pulse 243 may be delivered at adelay after the expiration of pacing interval 245 that is equal to thecharging time 244 required to charge the holding capacitor up to thepacing voltage amplitude.

Control circuit 80 may be configured to detect a decreased intrinsicheart rate in response to expiration of a single pacing interval 245without as sensed intrinsic event. In response to detecting thedecreased heart rate that is slower than the pacing rate based on nointrinsic event being sensed during the pacing interval 245, the controlcircuit 80 switches to charging without delay by charging the holdingcapacitor during each pacing interval as needed to maintain the holdingcapacitor(s) in a ready state for pacing pulse delivery. Pacing interval246 is started in response to delivering pacing pulse 243. Capacitorcharging may be initiated at the beginning or onset of pacing interval246 after detecting a decreased intrinsic heart rate based on at leastone expired pacing interval 245. Control circuit 80 may control thetherapy delivery circuit 84 to recharge the holding capacitor to thepacing voltage during charging time 247 following pacing pulse 243.

In addition to starting pacing interval 246, control circuit 80 maystart hysteresis interval 206 to begin monitoring for an increasedintrinsic heart rate as described above in conjunction with FIGS. 8C-8D.Control circuit 80 continues to charge the holding capacitor(s) duringeach pacing interval as needed according to the charging without delaymode in order to maintain the capacitor charge within a specifiedtolerance of the pacing voltage amplitude during each pacing intervaluntil an increased intrinsic heart rate is detected. Control circuit 80may enable power source 98 to provide power to the HV therapy circuit 83or LV therapy circuit 85 for charging selected holding capacitor(s)until an increased intrinsic heart rate is detected. Control circuit 80may compare a capacitor charge signal from the selected HV therapycircuit 83 or LV therapy circuit 85 at the start of pacing interval 246and each pacing interval thereafter until an increased intrinsic heartrate is detected again. If the capacitor charge signal indicates thatthe holding capacitor charge is less than a tolerance below the pacingvoltage amplitude, control circuit 80 enables charging of the selectedholding capacitor(s) as needed during pacing interval 246.

In FIG. 9A, the decreased intrinsic heart rate is detected by controlcircuit 80 in response to a single expired pacing interval for thepurposes of switching back to charging without delay. In other examples,more than one expired pacing interval may be required for detecting adecreased intrinsic heart rate to cause control circuit 80 to change thetiming of capacitor charging. For example, capacitor charging may occurupon expiration of the pacing interval for two or more pacing cyclesresulting in pacing pulses being delivered at the pacing interval plus adelay interval equal to the required charging time for the two or morepacing cycles. A decreased intrinsic heart rate may be detected inresponse to a predetermined number of consecutive or non-consecutive (Xout of Y) expired pacing intervals.

To illustrate, if a decreased intrinsic heart rate is detected inresponse to three consecutively expired pacing intervals, up to threepacing pulses may be delivered at a rate less than the pacing ratecorresponding to pacing interval 245. The actual pacing rate for thefirst three pacing pulses may be the rate corresponding to the pacinginterval 245 plus the charging time 244 required to charge the holdingcapacitor to the pacing voltage amplitude. For instance, the pacinginterval 245 may be set to 1.5 seconds for a lower pacing rate of 40pulses per minute. The charge time 244 may average 0.5 seconds,resulting in an actual pacing rate of 30 pulses per minute for the threepacing cycles leading up to satisfaction of the decreased heart ratedetection criteria. After the third pacing pulse, the control circuit 80may detect a decreased intrinsic heart rate based on decreased heartrate criteria and re-enable the therapy delivery circuit 84 to chargethe holding capacitor during each pacing interval without delay so thatsubsequent pacing pulses are each delivered at the expiration of theprogrammed pacing interval without delay

In some instances, a sensed event signal could be received by controlcircuit 80 during the charging time 244. In this case, the pacing pulse243 would be inhibited and capacitor charging could be terminated orallowed to continue. If a required number of expired pacing intervalshas been reached for decreased intrinsic heart rate criteria to be met,control circuit 80 may switch back to the charging without delay modeeven if a sensed event signal was received during the charging time,after the pacing interval expired, causing the pacing pulse to beinhibited. As such, a required number of pacing intervals for detectinga decreased intrinsic heart rate may be reached without requiring thesame number of delivered pacing pulses. The number of delivered pacingpulses may be less than the number of expired pacing intervals due tosensed event signals being received during the capacitor charging.

FIG. 9B is a timing diagram of a method for controlling holdingcapacitor charging during a decreasing intrinsic heart rate according toanother example. In some examples, capacitor charging is withheld forthe entire pacing interval according to the delayed capacitor chargingmode, as shown in FIG. 9A. In other examples, capacitor charging iswithheld for a portion of the pacing interval but may be started duringthe pacing interval after a charging delay interval. In the example ofFIG. 9B, a capacitor charging delay interval 256 is set in response tosensed event signal 251, along with starting pacing interval 255. Inresponse to detecting an increased heart rate based on the hysteresisinterval as described above in conjunction with FIGS. 8C or 8D, controlcircuit 80 may withhold capacitor charging during the pacing interval255 until after the capacitor charging delay interval 256 expires,according to the delayed charging mode. If a sensed event 252 occursduring the capacitor delay interval 256, holding capacitor charging iswithheld and the capacitor charging delay interval is restarted asinterval 258. The pacing interval is restarted as interval 257 inresponse to the sensed event signal 252.

Charging delay interval 258 expires before the next sensed event signal253. Capacitor charging is initiated as indicated by charging time 254in response to charging delay interval 258 expiring. In response toreceiving a sensed event signal 253, capacitor charging 254 may beterminated since the scheduled pacing pulse is inhibited and the pacinginterval is restarted as pacing interval 259. In other examples,charging to the pacing voltage amplitude may be completed duringcharging time 254 (which may extend past sensed event signal 253 andinto the next pacing interval 259). The next charging delay interval 260is started along with the pacing interval 259 in response to the sensedevent signal 253.

If charging delay interval 260 expires, control circuit 80 is configuredto control the therapy delivery circuit 80 to initiate capacitorcharging 254. If the pacing interval 259 expires, pacing pulse 264 isdelivered. A new pacing interval 261 is started and a hysteresisinterval 270 may be set in response to delivered pacing pulse 264 foruse in detecting an increased intrinsic heart rate again as describedabove, e.g., in conjunction with FIG. 7C or 7D. In some instances,pacing pulse 264 may be delivered upon pacing interval expirationwithout delay if the holding capacitor is fully charged to the pacingvoltage amplitude by the time the pacing interval 259 expires. In otherinstances, capacitor charging may be incomplete upon expiration ofpacing interval 259, and pacing pulse 264 may be delivered at a shortdelay after expiration of the pacing interval 259 as required tocomplete capacitor charging.

In some examples, control circuit 80 may be configured to determine anestimated capacitor charging time based on the pacing voltage amplitudeand capacitance of the holding capacitor. In other examples, controlcircuit 80 may be configured to determine the capacitor charging timebased on charging history. For instance, the time interval from thestart of capacitor charging following a delivered pacing pulse until acharge completion signal is received from the therapy delivery circuit84 may be determined by control circuit 80. This time interval, e.g.,charging interval 247 of FIG. 9A, or an average of multiple chargecompletion time intervals obtained in this manner during multiplecapacitor charging occurrences, may be determined as the capacitorcharging time.

Control circuit 80 may be configured to set the capacitor charging delayinterval 260 based on the pacing interval 259 and the calculated ormeasured capacitor charging time. The capacitor charging delay interval260 may be determined as the difference of the pacing interval 259 andthe determined capacitor charging time. In some examples, the capacitorcharging delay interval 260 may be the difference between pacinginterval 259 and the determined capacitor charging time less a pacingsafety interval (which may be set to zero) to promote charge completionprior to expiration of pacing interval 259 to avoid any delay of pacingpulse 264.

In the example shown in FIG. 9B, control circuit 80 controls the therapydelivery circuit 84 to recharge the holding capacitor during chargingtime 266 after delivery of pacing pulse 264. Expiration of a singlepacing interval 259 may cause control circuit 80 to switch to thecharging without delay mode to restore the holding capacitor charge tothe pacing voltage during each pacing interval without waiting for acapacitor charging delay interval to expire. In other examples, apredetermined number of pacing intervals may be required to expirebefore reverting back to charging without delay during each pacinginterval. For instance, charging may be performed after the capacitorcharging delay interval after two or more consecutive or non-consecutivepacing pulses, e.g., after 3 consecutive pacing pulses or after 3 pacingpulses out of 5 consecutive cardiac cycles, before switching to chargingwithout delay, without setting the capacitor charging delay interval.

FIG. 9C is a timing diagram of another example for controlling holdingcapacitor charging during a decreasing intrinsic heart rate. In theexample of FIG. 9B, control circuit 80 detects a decreased intrinsicheart rate based on a predetermined number of expired pacing intervals.In other examples, control circuit 80 may be configured to detect adecreased intrinsic heart rate in response to a predetermined number ofexpired capacitor charging delay intervals. The decreased intrinsicheart rate detection criteria may be determined to be satisfied based onexpired capacitor charging delay intervals, even if no pacing intervalshave expired.

As shown in FIG. 9C and described above in conjunction with FIG. 9B, asensed event 252 during a capacitor charging delay interval 256 causescapacitor charging to be withheld. Expiration of a capacitor chargingintervals 258 and 260 without sensed intrinsic events results incapacitor charging 254 to top off the holding capacitor charge at thepacing voltage amplitude. In the example of FIG. 9B, capacitor charging254 was terminated in response to a sensed event signal 253 during thecapacitor charging. In the example of FIG. 9C, capacitor charging 254continues after sensed event signal 253 to complete charging to thepacing voltage amplitude.

Sensed events 253 and 282 during the capacitor charging 254 and prior toexpiration of respective pacing intervals 257 and 288 cause scheduledpacing pulses to be inhibited. In response to the expiration of apredetermined number of charging delay intervals, two in this example(258 and 260), control circuit 80 detects a decreased intrinsic heartrate and switches the capacitor charging mode to charging without delayafter sensed event signal 282. A capacitor charging delay interval isnot started concomitantly with pacing interval 290. Capacitor chargingmay be completed as needed to top-off the capacitor charge followingsensed event 282 and during the subsequent pacing interval 290. The nextevent is a sensed event 284 during pacing interval 290. Capacitorcharging occurs without delay during pacing interval 292 as indicated bycharge time 286. A hysteresis interval 270 may be started following thesensed event 284 concomitantly with pacing interval 292 to facilitatedetection of increased intrinsic heart rate criteria being satisfied.

In this example, if a sensed intrinsic event occurs during the capacitorcharging delay interval, charging is withheld because the intrinsicheart rate is faster than the rate corresponding to the capacitorcharging delay interval. If a predetermined number of capacitor chargingdelay intervals expire, however, the intrinsic heart rate may bedecreasing toward a potential need for pacing. The control circuit 80may detect a decreased intrinsic heart rate in response to apredetermined number of expired capacitor charging delay intervals andswitch to charging without delay in response to detecting the decreasedintrinsic heart rate. As such, switching to capacitor charging withoutdelay may occur before the intrinsic heart rate falls below theprogrammed pacing rate and before any pacing pulses are delivered.

FIG. 10 is a flow chart 300 of a method for controlling holdingcapacitor charging according to another example. At block 302, controlcircuit 80 controls the therapy delivery circuit 84 to charge theselected holding capacitor(s) for delivering pacing pulses after acapacitor charging delay interval, which may be equal to or less thanthe pacing interval currently being used for scheduling a pacing pulse.Operations performed at block 302 may include determination of acalculated or measured capacitor charge completion time interval usedfor setting the capacitor charging delay interval. Control circuit 80withholds capacitor charging in response to receiving a sensed eventsignal during the capacitor charging delay interval, and the scheduledpacing pulse is inhibited, e.g., as shown in FIGS. 9B and 9C. Thecapacitor charging delay interval and the pacing interval are restarted.In other examples, the pacing interval is restarted after each pacingpulse and sensed event signal and capacitor charging is delayed until apacing interval expires without setting a separate capacitor chargingdelay interval, e.g., as in FIG. 9A.

Control circuit 80 may detect a decreased intrinsic heart rate at block304 based on a threshold number of expired pacing intervals, e.g., as inFIGS. 9A and 9B. The number of expired pacing intervals may be greaterthan the number of delivered pacing pulses since a sensed event afterthe pacing interval expiration may occur during delayed charging,causing the pacing pulse to be inhibited. In other examples, controlcircuit 80 may detect a decreased intrinsic heart rate in response to apredetermined number of expired charging delay intervals, e.g., as shownin FIG. 9C. In response to detecting the decreased intrinsic heart rate,control circuit 80 switches to controlling the therapy delivery circuit84 to charge the holding capacitor used for pacing without delay atblock 306, e.g., without setting the capacitor charging delay interval.Re-charging or top-off charging to maintain the holding capacitor chargeat the pacing voltage amplitude during the pacing interval may occur atthe beginning of a pacing interval or throughout the pacing interval setafter a delivered pacing pulse or sensed event.

At block 308, control circuit 80 may determine the rate or slope of thedecrease in the intrinsic heart rate. For example, the control circuit80 may determine the rate of decrease over a predetermined number ofheart beats or a predetermined time interval leading up to the time atwhich the decreased heart rate criteria were satisfied. If the heartrate decrease occurs abruptly, the decreased intrinsic heart ratedetection criteria may be adjusted at block 310 to enable detection ofthe decreased heart rate in fewer cardiac cycles, e.g., as few as onecardiac cycle. If the rate of decrease is relatively slow, the decreasedintrinsic heart rate detection criteria may be adjusted at block 310 byincreasing the number of expired capacitor charging delay intervalsand/or expired pacing intervals required to detect the decreasedintrinsic heart rate before switching from delayed capacitor charging tocharging the holding capacitor(s) without delay.

Control circuit 80 continues to control the therapy delivery circuit 84to charge the holding capacitor without delay during each pacinginterval to maintain the holding capacitor in a ready state until anincreased intrinsic heart rate is detected at block 312. As describedabove, criteria for detecting an increased intrinsic heart rate mayrequire one or more cardiac cycles having a sensed event signaloccurring within the hysteresis interval.

When increased intrinsic heart rate criteria are met at block 312,control circuit 80 may be configured to determine the rate or slope ofthe increase in the intrinsic heart rate at block 314. The rate ofincrease may be determined over a pre-determined time interval or numberof cardiac cycles. Based on the rate of increase, control circuit 80 mayadjust the criteria for detecting the increased intrinsic heart rate atblock 316. If the rate of increase occurs rapidly, the control circuitmay set a relatively low number of sensed events occurring at or abovethe hysteresis interval rate as a requirement for detecting theincreased intrinsic heart rate. Alternatively or additionally, thehysteresis interval may be set to a relatively longer interval, up tothe pacing interval, to promote earlier detection of an increasingintrinsic heart rate. By adjusting the increased intrinsic rate criteriain a patient whose heart rate recovers rapidly, battery charge isconserved by switching back to delayed capacitor charging earlier.

If the rate of increase occurs gradually, e.g., with intermittent pacingand sensing or sensing near the pacing rate for a sustained timeinterval, the hysteresis interval may be adjusted to a relativelyshorter interval and/or the required number of sensed hysteresisinterval events may be reduced. By adjusting the increased intrinsicrate detection criteria, switching back and forth between delayedcapacitor charging and charging without delay may be avoided and anypacing delays due to delayed capacitor charging may be avoided while theheart rate is gradually increasing.

After adjusting the increased rate detection criteria based on the rateof increase, the control circuit 80 operates to control the therapydelivery circuit 84 to delay charging of the holding capacitor duringpacing intervals following sensed events and pacing pulses. Capacitorcharging is delayed by withholding capacitor charging until a pacinginterval expires, e.g., as shown in FIG. 9A, or until a capacitorcharging delay interval expires, e.g., as shown in FIGS. 9B or 9C.Capacitor charging is delayed until a decreased intrinsic heart rate isdetected according to adjusted decreased rate detection criteria. It isto be understood that while adjustment of both decreased rate detectioncriteria and increased rate detection criteria are indicated in FIG. 10based on determining the respective rate of decrease and rate ofincrease of intrinsic heart rate changes, the control circuit 80 may beconfigured to automatically adjust only the decreased intrinsic heartrate detection criteria, adjust only the increased intrinsic heart ratedetection criteria, both or neither.

FIG. 11 is a flow chart 320 of a method for controlling the capacitorcharging mode based on the rate or slope of a change in heart rate. Atblock 321, ICD 14 may be delivering pacing pulses during a sustained runof pacing due to expired pacing intervals without sensed intrinsicevents. The control circuit 80 is controlling the therapy deliverycircuit 84 to charge the holding capacitor(s) without delay.

At block 322, an intrinsic event is sensed, causing a scheduled pacingpulse to be inhibited. The sensed event interval is determined at block323. At block 324, the slope of the heart rate over a predetermined timeinterval or a predetermined number of cardiac cycles is determined. Forexample, five to ten of the most recent sensed event intervals may beused to determine the slope of the heart rate change at block 324. Ifthe intrinsic heart rate is increasing rapidly, e.g., due to increasedpatient activity, capacitor charging may be withheld since thelikelihood of a pacing interval expiring is now greatly reduced. Controlcircuit 80 compares the slope of the intrinsic heart rate change overthe predetermined time interval or number of sensed intrinsic events toa threshold slope at block 325. This threshold slope is a positive slopecorresponding to a relatively rapid rise in intrinsic heart rate.

If the required time interval or number of sensed event intervals fordetermining the slope at block 324 has not been reached, the slope ofthe intrinsic heart rate change will be less than the slope threshold atblock 325. In that case, capacitor charging without delay continues atblock 326. The process returns to block 322 to wait for the next sensedevent.

If the required time interval or number of sensed event intervals hasbeen reached to enable slope determination at block 324, but the slopeis less than the slope threshold at block 325, the control circuit 80continues to control capacitor charging without delay at block 326. Theintrinsic heart rate may be increasing but may not be increasing rapidlyenough or may not be increasing monotonically to justify switching fromthe charging without delay mode to the delayed charging mode. Theprobability of a pacing interval expiring is still high enough towarrant maintaining the holding capacitor in a ready state for pacing.

If the slope of the intrinsic heart rate change determined at block 324is equal to or greater than the slope threshold at block 325, controlcircuit 80 switches the capacitor charging mode to delayed capacitorcharging at block 327, e.g., by setting a capacitor charging delayinterval as described previously or withholding capacitor chargingduring all or a portion of the next pacing interval. The slope thresholdapplied at block 325 may require that the intrinsic heart rate increasefrom the current pacing rate to 20 beats per minute faster than thepacing rate within one minute, for example, though other slopethresholds may be defined according to patient need.

After switching to delayed capacitor charging, the control circuit 80may continue to monitor the slope of the intrinsic heart rate change atblock 328. The slope may be compared to a rate drop threshold at block329. The rate drop threshold may be applied to the slope of theintrinsic heart rate over a predetermined time interval or predeterminenumber (Y) of sensed event intervals to determine if the intrinsic heartrate is rapidly decreasing, which increases the likelihood of a pacinginterval expiring. For example, the rate drop threshold may require thatthe slope of the intrinsic heart rate falls 30 beats per minute withinone minute. This rate drop threshold is a negative slope thresholdcorresponding to a relatively rapid decrease in heart rate. The dropthreshold and required time interval or number of sensed event intervalsfor determining the slope compared to the drop threshold may be defineddifferently than the respective slope and threshold determined and usedat blocks 324 and 325 for detecting a rapid rise in heart rate.

If the slope of the intrinsic heart rate change is greater than (e.g.,less negative than) the drop threshold, delayed charging of the holdingcapacitor(s) continues at block 327. The heart rate may be increasing (apositive slope), stable (zero slope), or slowly decreasing (smallernegative slope than the drop threshold), such that there is a relativelylow probability that a pacing interval will expire. If the slope is lessthan (more negative than) the drop threshold at block 329, indicating arapid decrease in heart rate and an increased likelihood of a pacinginterval expiring, control circuit 80 may switch to charging the holdingcapacitor(s) without delay at block 328.

It is to be understood that all or a part of the process of flow chart320 may be combined with the techniques described above in conjunctionwith FIGS. 6 through 9C such that control circuit 80 may switch thecontrol of capacitor charging in response to the slope of a heart ratechange crossing a slope threshold, e.g., being greater than thepositive, increasing slope threshold at block 325 and/or less than thenegative, drop slope threshold at block 329 before other increasedintrinsic heart rate detection criteria or decreased intrinsic heartrate detection criteria are met. When combined with the techniques forswitching capacitor charging mode based on other increased intrinsicheart rate detection criteria and decreased intrinsic heart ratedetection criteria, control circuit 80 may respond appropriately to bothrapid and relatively slower changes in intrinsic heart rate by switchingthe capacitor charging mode in a manner that conserves battery energy ofpower source 98 when expiration of a pacing interval is less likely andmaintains the holding capacitor charge in a ready state when a pacinginterval expiration is relatively more likely.

FIG. 12 is flow chart 400 of a method for enabling and disabling thecapacitor charging mode switching function based on detecting changes inpacing burden according to one example. ICD 14 may operate according tothe techniques described above in conjunction with any of FIGS. 6through 11 on a continuous basis. In other words, control circuit 80 maybe enabled to automatically switch between a charging without delay modeand a delayed charging mode based on intrinsic heart rate criteria atany time of day or independent of other physiological conditions. Inother examples, control circuit 80 may operate only in a defaultcapacitor charging mode, e.g., charging without delay, with the functionof switching between charging without delay and delayed charging modesdisabled (turned off). The function of charging mode switching may beenabled (turned on) and disabled (turned off) by control circuit 80 inresponse to an actual or predicted change in pacing burden.

In FIG. 12, ICD 14 initially operates in a default capacitor chargingmode at block 402. In the example shown, the default mode is chargingwithout delay during each pacing interval to maintain the capacitorcharge in a ready state for delivering a pacing pulse the next time apacing interval expires. The functions of detecting a change inintrinsic heart rate based on predefined criteria and switching betweencapacitor charging modes may be disabled or turned off during thisdefault charging mode.

At block 404, control circuit 80 may monitor one or more parameters fordetermining if decreased pacing burden criteria are satisfied. Adecreased pacing burden may be determined or predicted based on pacinghistory, time of day, and/or other physiological signals received fromsensors 90. In one example, decreased pacing burden criteria are metwhen the time of day is determined to be nighttime or a programmabletime of day that the patient is normally resting, goes to bed or isasleep. In response to the time of day reaching the programmed “night”time, control circuit 80 may enable or turn on the function of switchingcapacitor charging mode at block 406. Enabling the function of automaticswitching between charging modes does not necessarily mean thatswitching from one charging mode to the other occurs right away. Rather,after turning on the switching mode function, monitoring for detectingchanges in intrinsic heart rate may be performed to control switchingbetween the two charging modes. In order to actually switch from thecharging without delay mode that is currently active to the delayedcharging mode, increased intrinsic heart rate criteria need to be met,e.g., based on a sensed intrinsic event during at least one hysteresisinterval as described above in conjunction with FIGS. 8C and 8D.

After enabling the function of charging mode switching at block 406,control circuit 80 may monitor one or more parameters for determining ifincreased pacing burden criteria are met at block 408. The switchingfunction may be turned off or disabled again at block 410 if increasedpacing burden criteria are met. In the illustrative example of enablingcharging mode switching at block 406 in response to determining that thetime of day is “night” at block 404, corresponding to an expecteddecrease in pacing demand, control circuit 80 may determine thatincreased pacing burden criteria are met at block 408 in response to thetime of day reaching morning or a programmed time of day that thepatient is expected to wake up or become active. In a given patient, thepacing burden may be expected to be higher during active daytime hoursthan during night time hours. The increased pacing burden criteria aremet at block 408 based on the time of day and an expected higher pacingfrequency during daytime hours. It is recognized that the time of daythat is detected as meeting decreased pacing burden criteria and thetime of day that is detected as meeting increased pacing burden criteriamay be tailored according to a patient's individual habits and dailyroutine.

In other examples, the increased pacing burden criteria may bedetermined to be satisfied at block 408 based on an increase in theactual pacing burden determined as the number or percentage of deliveredpacing pulses during a predetermined time period, e.g., over at leastone hour or more. In response to the increased pacing burden criteriabeing met at block 408, control circuit 80 disables the function ofcharging mode switching at block 410. Control circuit 80 operates tocontrol the therapy delivery circuit 84 to charge the holdingcapacitor(s) according to the default pacing mode, which may be thecharging without delay pacing mode as indicated at block 402, withoutmonitoring for intrinsic heart rate changes and without switchingbetween charging modes.

Other criteria for detecting a decreased pacing burden at block 404 mayinclude an actual pacing burden falling below a predetermined threshold.For example, the pacing burden may be determined as the number of pacingpulses, the percentage of pacing pulses delivered out all cardiacevents, or the ratio of paced events to intrinsic sensed events during apredetermined time interval, e.g., one hour, two hours, four hours,eight hours, twelve hours, twenty-four hours, one week or other timeinterval. If the pacing burden falls below a pacing burden threshold,the decreased pacing burden criteria are met at block 404. Toillustrate, if fewer than 10% of all events, sensed and paced, aredelivered pacing pulses over the past twenty-four hours, control circuit80 may enable charging mode switching at block 406 such that delayedcapacitor charging may be performed when increased intrinsic heart ratecriteria are met.

Other examples of criteria for determining that the decreased pacingburden criteria are met at block 404 may be based on a sensor signalreceived from sensors 90. For instance a decrease in patient activity ora decreased sensor indicated pacing rate as determined from an activitysensor signal or other indicator of decreased metabolic demand, such asdecreased respiratory minute volume, may be an indicator of decreasedpacing burden. Similarly, criteria for detecting an increased pacingburden may be satisfied at block 408 in response to detecting anincrease in patient activity or sensor indicated rate determined from apatient activity sensor, such as an accelerometer or an impedance sensorused to track respiratory minute volume as an indication of increasedmetabolic demand.

Other physiological sensor signals correlated to the patient'shemodynamic function may be used to determine that decreased pacingburden criteria are met at block 404 and/or increased pacing burdencriteria are met at block 408. For example a signal or metric derivedfrom a pressure sensor, oxygen saturation sensor, impedance sensor, orother physiological sensor may be determined and compared to a thresholdfor determining a need for increased cardiac output. The increasedpacing burden criteria may be satisfied based on a need to increase thecardiac output. For instance, blood pressure, tissue or blood oxygensaturation, or other parameter that is correlated to cardiac output maybe determined by control circuit 80 from a sensor signal and compared toa threshold at block 408. If the parameter indicates that cardiac outputis low, e.g., below a predetermined threshold, such that an increase incardiac output is needed, increased pacing burden criteria may bedetermined to be satisfied at block 408, and charging mode switching maybe disabled at block 410.

The decreased pacing burden criteria and the increased pacing burdencriteria may be defined differently such that different criteria, whichmay include different parameters and/or different thresholds applied torespective parameters, are used to determine when to enable and disablethe function of switching charging mode. For example, the time of daymay be used to detect satisfaction of decreased pacing burden criteriafor enabling capacitor charging mode switching while a sensor signalparameter, e.g., indicative of an increased metabolic demand or a needfor increased cardiac output, may be used to detect that increasedpacing burden criteria are met at block 408 for disabling charging modeswitching at block 410.

After charging mode switching is enabled at block 406, control circuit80 may operate according to any of the techniques described above fordetecting an increased intrinsic heart rate and switching to delayedcapacitor charging. After switching to delayed capacitor charging,control circuit may monitor for a decreased intrinsic heart rate andswitch back to capacitor charging without delay as long as the switchingfunction has not been disabled due to increased pacing burden criteriabeing met.

It is further contemplated that the default charging mode is the delayedcharging mode rather than the charging without delay mode as shown inFIG. 12. The capacitor charging mode switching function may be enabledand/or disabled based on an actual or predicted pacing burden changecrossing a pacing burden threshold. A patient receiving ICD 14 may beexpected to seldom require pacing. In that case, control circuit 80 maycontrol therapy delivery circuit 84 to delay capacitor charging until acharging delay interval or pacing interval expires without a sensedintrinsic event. A slow intrinsic heart rate that requires pacing mayoccur without switching to the charging without pacing mode as long asthe charging mode switching function remains disabled. Control circuit80 may monitor actual pacing burden based on the frequency of deliveredpacing pulses, the time of day, and/or one or more physiological signalsfrom sensors 90 for determining if increased pacing burden criteria aremet, e.g. as described above in conjunction with block 408. In responseto the increased pacing burden criteria being met, control circuit 80may enable switching between capacitor charging modes. After capacitorcharging mode switching is enabled, control circuit 80 monitors forintrinsic heart rate changes and may switch from delayed capacitorcharging to charging without delay in response to decreased intrinsicheart rate criteria being met.

In some examples, after enabling the switching between charging modes,the switching function may never be disabled again. For example, uponinitial implant, ICD 14 may operate according to a default capacitorcharging mode, either charging with delay or delayed charging, withswitching between the two modes disabled based on the anticipated pacingneeds of the patient. Upon detecting that a change in pacing burden hasoccurred or is expected to occur, using any one or combination of thecriteria described above, capacitor charging mode switching is enabled.After being enabled, control circuit 80 switches between the twocharging modes based on detecting increased and decreased intrinsicheart rates according to predetermined criteria using any of theexamples given herein. Capacitor charging mode switching may remainenabled without ever being automatically disabled by control circuit 80,e.g., for the remaining life of the ICD 14 or until manuallyreprogrammed by a user.

FIG. 13 is a flow chart 450 of a method for controlling capacitorcharging by ICD 14 based on different pacing therapies according to oneexample. Control circuit 80 may start a pacing interval at block 452 inresponse to a sensed event signal or a delivered electrical stimulationpulse or other determination of a need for a pacing therapy. In somecases, the delivered stimulation pulse may be a CV/DF shock pulse inwhich case the pacing interval may be post-shock pacing interval forpreventing post-shock asystole. If the pacing interval is a post-shockpacing interval, as determined at block 454, the control circuit 80 maybe configured to disable delayed capacitor charging at block 460 inanticipation of a potential critical need for pacing without delay.Control circuit 80 may be configured to control the therapy deliverycircuit 84 to deliver post-shock pacing by charging the selected LV orHV holding capacitor(s) to the pacing voltage amplitude during eachpost-shock pacing interval, e.g., starting at the beginning of eachpacing interval and in some instances throughout the pacing interval asneeded to maintain the holding capacitor charge at the programmed pacingvoltage amplitude.

In other cases, the pacing interval started at block 452 may be startedin response to a sensed event signal at the time of a tachyarrhythmiadetection. In this case, the pacing interval may be an ATP interval setto control the delivery of a series of ATP pulses. If the pacinginterval is an ATP pacing interval, control circuit 80 may be configuredto disable delayed charging of the holding capacitor at block 460.Capacitor charging is performed during each ATP pacing interval withoutdelay to promote accurate timing of ATP pacing pulses and successfultachyarrhythmia termination.

At other times, the pacing interval started at block 452 may be abradycardia pacing interval, e.g., a VVI pacing interval, started inresponse to a delivered bradycardia pacing pulse or R-wave sensed eventsignal. If the pacing interval was started as a bradycardia pacinginterval (“no” branch of block 454) and the precipitating event is asensed event signal that is detected as a premature ventricularcontraction (PVC) by the control circuit 80, the control circuit 80 maydisable delayed capacitor charging at block 460 for one cycle. Capacitorcharging may be performed during the pacing interval started in responseto a sensed event identified as a PVC without delay as needed to top-offthe capacitor charge to the pacing voltage amplitude. In this way, thetherapy delivery circuit 84 is ready to deliver a pacing pulse inanticipation of a long, compensatory pause following the PVC. Controlcircuit 80 may be configured to detect a sensed R-wave as a PVC based onthe sensed event interval (RR interval) since a most recent precedingventricular event (paced or sensed) and/or whether or not an atrialP-wave was sensed prior to the sensed R-wave during the RR intervalending on the sensed R-wave. For example, an R-wave sensed event signalthat is received at an RRI that is less than a PVC detection thresholdinterval may be detected as a PVC at block 406.

In other examples, PVCs may be ignored for the purposes of controllingchanges in the capacitor charging state. For example, a short sensedevent interval determined to end on a sensed event signal identified asa PVC may be ignored in detecting an increased intrinsic heart rate. Along sensed event interval (the compensatory pause) following a sensedevent signal identified as a PVC may be ignored in detecting a decreasedintrinsic heart rate. In this way, a PVC or a run of PVCs will not alterthe capacitor charging state by causing capacitor charging modeswitching. If the control circuit 80 is presently operating to chargethe holding capacitor(s) used for generating pacing pulses withoutdelay, sensed event intervals immediately preceding and immediatelyfollowing a PVC are ignored for the purposes of detecting a change inintrinsic heart rate. No change to the capacitor charging without delayis made. Likewise, if the control circuit 80 is presently operating inthe delayed charging mode, the sensed event interval immediatelypreceding a sensed event identified as a PVC and the sensed eventinterval immediately following the PVC are ignored for the purposes ofdetecting a change in the intrinsic heart rate.

If the pacing interval started at block 452 is not an ATP pacinginterval or a post-shock pacing interval and is not started in responseto a sensed event signal that is detected as a PVC (“no” branch of block456), the control circuit 80 may operate according to the currentcapacitor charging control mode at block 458. If the control circuit 80is operating to delay capacitor charging until the expiration of acapacitor charging delay interval or until expiration of a pacinginterval, the capacitor charging is withheld during the pacing intervalthat was started at block 452 according to the delayed charging mode.Capacitor charging is delayed until the expiration of the pacinginterval or a capacitor charging delay interval as described inconjunction with FIGS. 9a -9 c. If the control circuit 80 has recentlydetected a decreased intrinsic heart rate and is operating to chargewithout delay, capacitor charging may be performed without delay, e.g.,at the beginning and/or throughout, the pacing interval started at block452 as needed to maintain the holding capacitor in a ready state forpacing pulse delivery.

Control circuit 80 may be configured to enable delayed capacitorcharging only during selected pacing therapies, e.g., during VVI pacing,so that pacing pulse delivery is not delayed during other pacingtherapies such as post-shock pacing and ATP when pacing pulse timing maybe critical. Depending on the types of electrical stimulation therapiesthat the ICD 14 or other IMD implementing the techniques disclosedherein is capable of, the control circuit 80 may be configured todisable delayed capacitor charging for one or more therapies and enabledelayed capacitor charging for one or more therapies.

FIG. 14 is a diagram of an IMD system 500 that may be configured tocontrol delayed capacitor charging for pacing therapy delivery accordingto another example.

IMD system 500 may include ICD 514 and intra-cardiac pacemaker 550. ICD514 is shown coupled to transvenous leads 510 and 520 in communicationwith the right atrium (RA) 502 and right ventricle (RV) 504,respectively, of heart 508. ICD 514 is shown as a dual-chamber pacemakerand cardioverter/defibrillator configured to sense cardiac signals anddeliver electrical stimulation pulses in RA 502 and RV 504. ICD 514includes a housing 515 enclosing electronic circuitry, e.g., a sensingcircuit, therapy delivery circuit, control circuit, memory, telemetrycircuit, other optional sensors, and a power source as generallydescribed in conjunction with FIG. 3 above. ICD 514 is shown implantedin a right pectoral position in FIG. 14, however it is recognized thatICD 514 may be implanted in other locations, e.g., in a left pectoralposition, particularly when ICD 514 includes cardioversion anddefibrillation capabilities using housing 515 as an active electrode.

ICD 514 has a connector assembly 517 for receiving proximal connectorsof RA lead 510 and RV lead 520. RA lead 510 may carry a distal tipelectrode 512 and ring electrode 514 for sensing atrial signals, e.g.,P-waves attendant to atrial depolarization, and delivering RA pacingpulses. RV lead 520 may carry pacing and sensing electrodes 522 and 524for sensing ventricular signals, e.g., R-waves attendant to RVdepolarization, and for delivering RV pacing pulses. RV lead 520 mayalso carry RV defibrillation electrode 526 and a superior vena cava(SVC) defibrillation electrode 528. Defibrillation electrodes 526 and528 are shown as coil electrodes spaced apart proximally from the distalpacing and sensing electrodes 522 and 524 and may be used for deliveringhigh voltage CV/DF shock pulses.

ICD 514 may be configured to provide dual chamber pacing in RA 502 andRV 504. In some examples, IMD system 500 may include an intracardiacpacemaker 550 positioned in left ventricle 506 for sensing leftventricular signals, e.g., R-waves attendant to left ventriculardepolarizations, and for delivering pacing pulses to left ventricle 506.IMD system 500 may be configured to deliver multi-chamber pacingtherapies such as cardiac resynchronization therapy (CRT). Intra-cardiacpacemaker 550 may be configured to deliver left ventricular pacingpulses to synchronize left ventricular contraction with RA and RVcontractions to promote a normal atrio-ventricular interval andcoordinated ventricular contractions. ICD 514 may be configured todeliver RA pacing pulses and RV pacing pulses as needed to prevent theheart rate from falling below a programmed lower pacing rate. In somepatients, occasional atrial bradycardia or AV conduction block may causeslowing of the intrinsic rate requiring pacing of the RA 502 and/or RV504. During CRT, however, left ventricular pacing by intra-cardiacpacemaker 550 may occur on a beat-by-beat basis, whether RA 502 and RV504 are being paced or sensed, for promoting optimal heart chambersynchrony.

Intra-cardiac pacemaker 500 may include housing based electrodes 552 and554 for sensing cardiac signals in the left ventricle 506 and deliveringleft ventricular pacing pulses. Pacemaker 500 may include a sensingcircuit and therapy delivery circuit that includes at least one pacingchannel in a low voltage therapy circuit including a low voltagecharging circuit, a holding capacitor, and an output capacitor, e.g., asgenerally described in conjunction with FIG. 5, for generating anddelivering pacing pulses to the left ventricle. In some examples,intra-cardiac pacemaker 550 includes a control circuit configured toperform the methods disclosed herein in conjunction with theaccompanying flow charts for controlling holding capacitor charging. Forexample, the control circuit of intra-cardiac pacemaker 550 may switchbetween delayed holding capacitor charging following detection of anincreased intrinsic heart rate and charging without delay during apacing interval following detection of a decreased intrinsic heart rate,respectively, as described above. Some patients may require sustained orprolonged episodes of left ventricular pacing in order to promote heartchamber synchrony. In this case, intra-cardiac pacemaker 550 may beconfigured to perform capacitor charging during each pacing intervalwithout delay.

For example, patient 512 may be dependent on LV pacing by intra-cardiacpacemaker 550 for promoting heart chamber synchrony, but RA pacing andRV pacing may be seldom required. In this situation, ICD 514 may beconfigured to switch between delayed capacitor charging and chargingwithout delay modes in one or both of the RA and RV pacing channels ofICD 514 to conserve battery charge. For example, when increasedintrinsic rate criteria are satisfied by sensed events (P-waves) in theRA and/or sensed events (R-waves) in the RV, the holding capacitorscorresponding to the RA pacing channel and the RV pacing channel may becharged according to the delayed charging mode.

ICD 514 may include at least two pacing channels of a low voltagetherapy circuit, e.g., any two of channels 342, 344 and 346 of lowvoltage therapy module 85 as shown in FIG. 5, for providing pacing to RA502 and RV 504. For example, RA electrodes 516 and 518 may be coupled topacing channel 346 of low voltage therapy circuit 85 for delivering RApacing pulses. RV electrode 522 and 524 may be coupled to pacing channel344 of low voltage therapy circuit 85 for delivering RV pacing pulses.The control circuit of ICD 514 may be configured to delay capacitorcharging in one or both of the RA pacing channel and the RV pacingchannel based on increased intrinsic rate criteria being satisfied inthe respective heart chamber.

For instance, with reference to the low voltage therapy circuit 85 ofFIG. 5, charging of low voltage holding capacitor 358 may be delayed inresponse to detecting an increased intrinsic atrial rate based on therate of sensed P-waves by the sensing circuit of ICD 514. A hysteresisinterval may be set after each sensed P-wave for detecting an increasedintrinsic atrial rate in response to a threshold number of cardiaccycles in which a sensed P-wave occurs during the hysteresis interval.ICD 514 may switch to charging the holding capacitor 358 of the pacingchannel 346 being used as the atrial pacing channel to the chargingwithout delay mode in response to detecting a decreased intrinsic atrialrate based on one or more expired atrial pacing intervals.

The control circuit of ICD 514 may set an AV pacing interval followingeach atrial pacing pulse and sensed P-wave in RA 502 for controlling thetiming of pacing pulses delivered to RV 504 by pacing channel 344 usedas an RV pacing channel coupled to electrodes 522 and 524. Additionallyor alternatively, the control circuit of ICD 514 may set a VV pacinginterval following each RV pacing pulse and each R-wave sensed in RV 504for controlling the timing of RV pacing pulses delivered by pacingchannel 344. The control circuit of ICD 514 may delay charging of lowvoltage holding capacitor 356 until after a capacitor charging delayinterval or after expiration of an expired AV or VV pacing interval inresponse to detecting an increased intrinsic ventricular rate based on athreshold number of cardiac cycles having an R-wave sensed in the RV 504during a hysteresis interval. The control circuit of ICD 514 may switchto charging without delay in response to the expiration of a thresholdnumber of pacing intervals and/or charging delay intervals. In someexamples, control of capacitor charging using delayed charging afterdetecting an increased intrinsic rate is used only for controllingcharging during AA or VV pacing intervals. Delayed capacitor chargingmay not be used for controlling capacitor charging associated with AVpacing intervals since AV pacing intervals are relatively shorter thanAA and VV pacing intervals and a ventricular pacing pulse delivered at along AV interval due to delayed capacitor charging may be undesirable.

In other examples, a pacemaker or ICD incorporating the techniquesdisclosed herein may be a single chamber pacemaker or ICD coupled to asingle transvenous lead or a multi-chamber pacemaker or ICD coupled tothree transvenous leads including a RA lead, RV lead and a coronarysinus lead for sensing and stimulating in RA 502, RV 504 and LV 506,respectively. In other examples of an IMD system performing thetechniques disclosed herein, the intra-cardiac pacemaker 550 may beincluded in an implantable system with ICD 14 and extra-cardiovascularlead 16 shown in FIG. 1A. Intra-cardiac pacemaker 550 may be placed inany atrial or ventricular chamber and control capacitor charging usingthe methods described above for delaying capacitor charging in responseto increased intrinsic heart rate criteria being satisfied.

FIG. 15 is a flow chart 600 of a method for controlling holdingcapacitor charging according to yet another example. Control circuit 80may control the therapy delivery circuit 84 to charge the holdingcapacitor(s) used for pacing pulse delivery in response to a pacinginterval expiring at block 602 during a delayed capacitor charging mode.Capacitor charging is withheld for the pacing interval by delayingcharging until after the pacing interval expires.

If decreased intrinsic heart rate criteria are satisfied at block 604,e.g., using any of the decreased intrinsic heart rate criteria describedabove such as a threshold number of expired pacing intervals or a slopeof the heart rate change being less than (more negative than) a dropthreshold, control circuit 80 may switch to charging the holdingcapacitor(s) during the pacing interval but after expiration of thecapacitor charging delay interval at block 606. In this way, charging isonly performed when the intrinsic heart rate is slower than the ratecorresponding to the capacitor charging delay interval. A sensed eventduring the capacitor charging delay interval causes pacing pulseinhibition and charging is withheld. When increased intrinsic heart ratedetection criteria are met, as determined at block 608, control circuit80 switches back to delayed charging at block 602 and withholdscapacitor charging until a pacing interval expires. Charging occurs whenthe heart rate is slower than the pacing rate.

This method of charging only when the intrinsic rate is less than therate corresponding to capacitor charging delay interval may be used inan IMD and electrode system having relatively low pacing capturethresholds. For example, intracardiac pacemaker 550 having housing-basedelectrodes in close proximity or in intimate contact with theendocardium or ICD 514 having transvenous leads with endocardialelectrodes are expected to have relatively low pacing capturethresholds. The time required to charge a holding capacitor to theprogrammed pacing voltage amplitude may be relatively short such thatcharging may occur after a charging delay interval, even when thelikelihood of a pacing interval expiring is increased based on decreasedintrinsic rate criteria being met. When the likelihood of a pacinginterval expiring is relatively lower, based on increased intrinsicheart rate criteria being met, charging may occur after the pacinginterval expires without resulting in a clinically significant delay topacing pulse delivery. In this example, charging after the capacitorcharging delay interval may be considered the “charging without delaymode” since charging is still performed during the pacing interval.Charging after the pacing interval expires may be considered the“delayed charging mode” since charging is withheld and delayed untilafter the pacing interval expires.

Methods described in conjunction with flow diagrams presented herein maybe implemented in a non-transitory computer-readable medium thatincludes instructions for causing a programmable processor to carry outthe methods described. A non-transitory computer-readable mediumincludes but is not limited to any volatile or non-volatile media, suchas a RAM, ROM, CD-ROM, NVRAM, EEPROM, flash memory, or othercomputer-readable media, with the sole exception being a transitory,propagating signal. The instructions may be implemented by processingcircuitry hardware as execution of one or more software modules, whichmay be executed by themselves or in combination with other software.

Various aspects of the techniques may be implemented within one or moreprocessors, including one or more microprocessors, DSPs, ASICs, FPGAs,or any other equivalent integrated or discrete logic circuitry, as wellas any combinations of such components, embodied in programmers, such asphysician or patient programmers, electrical stimulators, or otherdevices. The term “processor” or “processing circuitry” may generallyrefer to any of the foregoing logic circuitry, alone or in combinationwith other logic circuitry, or any other equivalent circuitry.

In one or more examples, the functions described in this disclosure maybe implemented in hardware, software, firmware, or any combinationthereof. If implemented in software, the functions may be stored on, asone or more instructions or code, a computer-readable medium andexecuted by a hardware-based processing unit. Computer-readable mediamay include computer-readable storage media forming a tangible,non-transitory medium. Instructions may be executed by one or moreprocessors, such as one or more DSPs, ASICs, FPGAs, general purposemicroprocessors, or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto one or more of any of the foregoing structure or any other structuresuitable for implementation of the techniques described herein.

In addition, in some aspects, the functionality described herein may beprovided within dedicated hardware and/or software modules. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.Also, the techniques could be fully implemented in one or more circuitsor logic elements. The techniques of this disclosure may be implementedin a wide variety of devices or apparatuses, including an IMD, anexternal programmer, a combination of an IMD and external programmer, anintegrated circuit (IC) or a set of ICs, and/or discrete electricalcircuitry, residing in an IMD and/or external programmer.

Thus, IMD systems and methods for controlling holding capacitor chargingfor pacing therapy delivery have been presented in the foregoingdescription with reference to specific embodiments. In other examples,various methods described herein may include steps performed in adifferent order or different combination than the illustrative examplesshown and described herein. It is appreciated that various modificationsto the referenced embodiments may be made without departing from thescope of the disclosure and the following claims.

1. A method comprising: delivering a pacing pulse; starting a firstpacing interval corresponding to a pacing rate in response to thedelivered pacing pulse; charging a holding capacitor during the firstpacing interval according to a first charging mode; detecting anincreased intrinsic heart rate that is at least a threshold rate fasterthan the pacing rate from a sensed cardiac electrical signal; switchingfrom the first charging mode to a second charging mode in response todetecting the increased intrinsic heart rate; starting a second pacinginterval in response to a first intrinsic cardiac event sensed from thecardiac electrical signal; and withholding charging of the holdingcapacitor for at least a portion of the second pacing interval accordingto the second charging mode.
 2. The method of claim 1, wherein detectingthe increased intrinsic heart rate comprises detecting at least onesensed cardiac event from the cardiac electrical signal at a hysteresisrate that is greater than the pacing rate.
 3. The method of claim 1,further comprising: starting a capacitor charging delay interval inresponse to the first sensed intrinsic cardiac event; withholdingcharging of the holding capacitor until expiration of the capacitorcharging delay interval; and charging the holding capacitor in responseto expiration of the capacitor charging delay interval.
 4. The method ofclaim 3, further comprising: terminating the capacitor charging inresponse to a second sensed intrinsic cardiac event during the secondpacing interval and after the capacitor charging delay intervalexpiring.
 5. The method of claim 4, further comprising: detecting asecond intrinsic cardiac event during the capacitor charging delayinterval; and restarting the capacitor charging delay interval withoutcharging the holding capacitor in response to detecting the secondintrinsic cardiac event during the capacitor charging delay interval. 6.The method of claim 4, further comprising determining a capacitorcharging time; and setting the capacitor charging delay interval basedon a difference between the second pacing interval and the capacitorcharging time.
 7. The method of claim 1, further comprising: detectingthe increased intrinsic heart rate in response to a predetermined numberof consecutive intrinsic cardiac events sensed from the cardiacelectrical signal by the sensing circuit; starting a capacitor chargingdelay interval in response to detecting the increased intrinsic heartrate; and withhold charging of the holding capacitor until expiration ofthe capacitor charging delay interval.
 8. The method of claim 1, furthercomprising: starting a hysteresis interval; detecting the increasedintrinsic heart rate in response to detecting the first intrinsiccardiac event during the hysteresis interval; starting a capacitorcharging delay interval in response to detecting the increased intrinsicheart rate; withholding charging of the holding capacitor untilexpiration of the capacitor charging delay interval.
 9. The method ofclaim 1, further comprising: detecting a decreased intrinsic heart rate;disabling withholding of the capacitor charging in response to detectingthe decreased intrinsic heart rate.
 10. The method of claim 9, furthercomprising: withholding charging of the holding capacitor by setting acapacitor charging delay interval in response to the first intrinsiccardiac event, the capacitor charging delay interval less than thesecond pacing interval; and detecting the decreased intrinsic heart ratein response to a predetermined number of capacitor charging delayintervals expiring without sensing a cardiac event from the cardiacelectrical signal during the predetermined number of capacitor chargingdelay intervals.
 11. The method of claim 9, wherein: detecting thedecreased intrinsic heart rate comprises detecting the decreasedintrinsic heart rate in response to decreased intrinsic heart ratecriteria being satisfied; determining a rate of decrease of thedecreased intrinsic heart rate; and adjusting the decreased intrinsicheart rate criteria based on the rate of decrease.
 12. The method ofclaim 1, further comprising: setting the first pacing intervalcorresponding to a first pacing therapy; setting a third pacing intervalcorresponding to a second pacing therapy different than the first pacingtherapy; disabling withholding of the charging of the holding capacitorin response to setting the third pacing interval.
 13. The method ofclaim 1, further comprising: detecting a premature ventricularcontraction; and disabling withholding of the charging of the holdingcapacitor in response to detecting the premature ventricularcontraction.
 14. The method of claim 1, further comprising: detecting apremature ventricular contraction from the cardiac electrical signal;wherein detecting the increased intrinsic heart rate comprises ignoringthe premature ventricular contraction.
 15. The method of claim 1,further comprising: detecting the increased intrinsic heart rate inresponse to increased intrinsic heart rate criteria being satisfied;determining a rate of increase of the increased intrinsic heart rate;and adjusting the increased intrinsic heart rate criteria based on therate of increase.
 16. The method of claim 1, further comprising:controlling holding capacitor charging according to only one of thefirst charging mode or the second charging mode with switching betweenthe first and second charging modes disabled; determining an actualpacing burden over a predetermined time interval; comparing the actualpacing burden to a pacing burden threshold; and enabling switchingbetween the first charging mode and the second charging mode in responseto the actual pacing burden crossing the pacing burden threshold. 17.The method of claim 1, further comprising; monitoring a sensor signalcorrelated to a patient condition; detecting a change in expected pacingburden based on the sensor signal; and in response to detecting a changein the expected pacing burden, enabling switching between the firstcharging mode and the second charging mode.
 18. The method of claim 1,further comprising a sensor configured to produce a signal correlated toa patient condition; monitoring a sensor signal that is correlated to apatient condition: detecting an expected increase in pacing burden basedon the sensor signal; and switching from the second charging mode to thefirst charging mode in response to detecting the expected increase inpacing burden.
 19. The method of claim 1, further comprising:determining a slope of a change in rate of intrinsic cardiac eventssensed from the cardiac electrical signal; comparing the slope to aslope threshold; and switching between the first and second chargingmodes in response to the slope crossing the slope threshold.
 20. Themethod of claim 1, wherein the control circuit is further configured to:enable the switching between the first charging mode and the secondcharging mode based on a time of day.
 21. The method of claim 1,wherein: charging the holding capacitor comprises charging a highvoltage holding capacitor that is chargeable to acardioversion/defibrillation shock voltage amplitude to the pacingvoltage amplitude for delivering the pacing pulse; and withholding thecharging comprising withholding charging of the high voltage holdingcapacitor in response to detecting the increased intrinsic heart rate.22. The method of claim 1, wherein: charging the holding capacitorcomprises charging at least two low voltage holding capacitors to thepacing voltage amplitude for delivering the pacing pulse; andwithholding the charging comprises withholding charging of the at leasttwo low voltage holding capacitors in response to detecting theincreased intrinsic heart rate.
 23. The method of claim 1, furthercomprising: withholding charging of the holding capacitor according tothe second charging mode by withholding a comparison of a charge of theholding capacitor to the pacing voltage amplitude at a start of thesecond pacing interval; detect a decreased intrinsic heart rate; switchfrom the second charging mode back to the first charging mode inresponse to detecting the decreased intrinsic heart rate; start a thirdpacing interval in response to a second intrinsic cardiac event sensedfrom the cardiac electrical signal; and charging the holding capacitoraccording to the first charging mode during the third pacing intervalby: comparing a charge of the holding capacitor to a pacing voltageamplitude at a start of the third pacing interval, and charging theholding capacitor in response to the holding capacitor voltage beingless than the pacing voltage amplitude.
 24. The method of claim 1,further comprising: starting a capacitor charging delay interval;withholding charging of the holding capacitor according to the secondcharging mode by withholding a comparison of a charge of the holdingcapacitor to the pacing voltage amplitude until the capacitor chargingdelay interval expires; detecting a decreased intrinsic heart rate;switching from the second charging mode back the first charging mode inresponse to detecting the decreased intrinsic heart rate; starting athird pacing interval in response to a second intrinsic cardiac eventsensed from the cardiac electrical signal; and charging the holdingcapacitor according to the first charging mode during the third pacinginterval by: comparing a charge of the holding capacitor to a pacingvoltage amplitude throughout the third pacing interval, and charging theholding capacitor in response to the holding capacitor voltage beingless than the pacing voltage amplitude.
 25. The method of claim 1,further comprising: charging the holding capacitor during the firstpacing interval after a charging delay interval; and withholdingcharging of the capacitor until the second pacing interval expires inresponse to detecting the increased intrinsic heart rate.